the present invention will now be described by way of example only and with reference to the accompanying drawings wherein : [ 0042 ] fig1 shows the mean amino acid consumption or production profile ( pmol / embryo / h ) for 27 human embryos which arrested development at the 8 - cell stage . amino acids which were significantly consumed or produced are marked with an asterix . [ 0043 ] fig2 shows the mean amino acid consumption or production profile ( pmol / embryo / h ) for 22 human embryos at the compacting 8 - cell stage which developed to the blastocyst stage . amino acids which were significantly consumed or produced are marked with an asterix . [ 0044 ] fig3 is a superimposition of the data from fig1 and 2 and compares the amino acid consumption or production ( pmol / embryo / h ) for human embryos which arrested development at the 8 - cell stage ( shaded bars ) and human embryos which developed to the blastocyst stage ( unshaded bars ). t - tests were used to compare the consumption or production data for each amino acid for each group of arresting and developing embryos . a p - value of less than 0 . 05 was considered significant . data for the amino acids marked with the superscript a , b , c or d ( alanine , asparagine , glycine and lysine respectively ) are significantly different for the two sets of tests . [ 0045 ] fig4 is a superimposition of data to compare amino acid consumption or production ( pmol / embryo / h ) for human embryos from day 2 to day 3 post fertilisation which arrested development at the 8 - cell stage ( shaded bars ) and human embryos from day 2 to day 3 post fertilisation which developed to the blastocyst stage ( unshaded bars ). [ 0046 ] fig5 shows the sum of utilisation of five amino acids in developing and arresting human embryos from day 2 to day 3 post fertilisation . [ 0047 ] fig6 is a superimposition of data to compare amino acid consumption or production ( pmol / embryo / h ) for icsi embryos that resulted in pregnancy ( unshaded bars ) and that failed to result in pregnancy ( shaded bars ) from day 1 to day 2 of development . [ 0048 ] fig7 shows the sum of gly , ala and trp appearance for icsi embryos that resulted and failed to result in pregnancy , from day 1 to day 2 of development . in initial experiments , spare in vitro fertilised human embryos were individually placed in 4 μl drops of culture medium until they reached the blastocyst stage . embryos were cultured individually to mimic the conditions which are encountered by an embryo developing in vivo . the culture medium comprised 4 μl drops of ebss supplemented with 1 mm glucose , 5 mm l - lactate , 0 . 47 mm pyruvate and a physiological mixture of 20 amino acids . the individual concentrations of the amino acids were in the range 0 . 005 mm to 1 . 0 mm . during incubation , the concentration of 18 amino acids in the spent medium was simultaneously measured using high performance liquid chromatography ( hplc ) following derivatisation with o - phthaldialdehyde . the results were used to compile amino acid consumption and production profiles as illustrated in fig1 to 3 . in order to achieve accurate dilution of microlitre samples for use in hplc , an internal standard in the form of the non metabolisable amino acid ; d - alpha - aminobutyric acid , was introduced into the medium at a concentration of 1 in 49 parts per volume . this internal standard allowed the minute differences that occur in resting embryos , and which would otherwise be lost in the ‘ back - ground noise ’ to be picked up . the hplc peak attributable to the marker can be easily distinguished and was used to calculate the correct dilution . as shown in fig2 the only amino acids that were significantly depleted from the medium during the compacting 8 - cell to the morula stage were serine , arginine , isoleucine and leucine . alanine , aspartate , glutamate and tryptophan were significantly produced . alanine appearance increased from 10 . 87 ± 1 . 61 pmol / embryo / h for embryos cultured between the compacting 8 - cell to the morula stage compared to 13 . 90 ± 1 . 23 pmol / embryo / h for embryos cultured from the morula to blastocyst stage . embryos that arrested at the compacting 8 - cell stage produced significantly more alanine ( 17 . 72 ± 1 . 44 pmol / embryo / h ; p = 0 . 0028 ) than developing embryos of the same stage ( fig1 to 3 ). glutamate was also significantly produced over the morula to blastocyst transition . although it is known that an embryo will cause changes to the amino acid concentration of a culture medium in which it is incubated , the significant differences in the changes brought about by healthy and arresting embryos have previously gone unnoticed . it should be understood that the results of the embryo experiments are highly indicative of those which would be expected for an egg , karyoplast , stem cell , stem cell precursor or embryo made by nuclear transfer subjected to the same incubation conditions . accordingly , the change in concentration of at least one amino acid in the culture medium can be used to give an indication as to the viability of an embryo , egg , karyoplast , stem cell , stem cell precursor or embryo made by nuclear transfer . this represents a major advance in the selection of embryos or eggs for subsequent implantation and the selection of karyoplasts , stem cells , stem cell precursors or embryos made by nuclear transfer for genetic manipulation . the increased production of alanine by arresting embryos for example is surprising and counter - intuitive . if alanine is used as a route to sequester waste nitrogen and is formed by the embryo to prevent the build - up of toxic ammonium ions , it might be expected that the healthiest embryos would be most metabolically active and produce higher quantities of alanine as compared with arresting embryos . surprisingly , the results show that healthiest embryos are metabolically ‘ quieter ’ and that embryos which fail to develop turnover comparatively more protein with the amino groups being exported into the culture medium as alanine . the results indicate that the human pre - implantation embryo is able to use amino acids selectively at different stages of development and that the appearance of alanine in the medium may be used as a potential marker of the viability of an embryo , egg , karyoplast , stem cell , stem cell precursor or embryo made by nuclear transfer . the method of the present invention could be used in pre - implantation screening for diseases such as phenylketonuria ( pku ), cystic fibrosis and other such genetic or chromosomal abnormalities . all babies in the uk are currently tested for pku a few days after birth by measuring the phenylalanine level in the blood using chromatography or a bacterial growth test . it is likely that those embryos or eggs which are predestined to pku are characterised by a different amino acid profile compared with that produced by a healthy embryo or egg . accordingly the method of the present invention carries huge potential for future genetic screening programs . the method of the present invention may also have immense value in sex determination whereby embryos of different sex may be characterised by a certain amino acid profile . it should be understood that the method of the invention has wide reaching applications and is not limited to the use of a human embryo , egg , karyoplast , stem cell , stem cell precursor or embryo made by nuclear transfer . a typical method in cattle breeding is the administration of egg inducing hormones to a cow of high genetic merit followed by natural insemination leading to the production of about 6 to 8 embryos in the uterus of the animal . the embryos are then ‘ flushed ’ from the cow and transferred singly to lower grade animals for subsequent development . due to the risks of improper flushing , this technique is unethical for use in man . alternatively , egg production in a valuable cow is hormonally induced and following retrieval ( ovum pick - up ), the eggs are artificially inseminated using high quality sperm and cultured to the blastocyst stage before subsequent transferral back to a recipient . cattle embryos may also be generated from oocytes derived from abattoir ovaries . such cattle breeding programmes are of multi - national concern and any selection method by which the most viable eggs or embryos for transfer can be selected represents a major industrial advance . other animals to which this technology may be applicable include sheep , pigs , all domestic animals and rare and threatened species . the cloning technology used in production of ‘ dolly ’ was met by low success rates with 276 previously failed attempts . the ‘ dolly ’ programme also involved considerable expenditure of research effort in the attempt to generate appropriate sheep cells to produce transgenic animals . the method of the present invention now provides a rational approach to the selection of the most viable cell for use in subsequent embryo or egg transfer and to the selection of karyoplasts , putative stem cell populations , stem cell precursor populations , stem cell populations and embryos made by nuclear transfer . it represents a major advance in farm animal improvement techniques which involve micro - manipulation , nuclear transfer and the addition of genetic constructs . it should be understood that the amino acid profile for a cell of a particular organism may be highly discrete ; differing in detail as to which particular amino acids are consumed and produced and also which particular amino acids are consumed and produced in arresting and developing cells when compared with that profile generated for a cell of a different organism . although the results suggest that alanine may be used as a potential marker for the viability of human cells , further investigations may reveal that other amino acids are more suitable markers for other species . indeed , further investigation of the human embryo reveal that additional amino acids besides alanine are also suitable for use as a selection marker . the initial study was based upon human embryos from day 3 to day 4 post insemination . as shown in fig3 lysine is produced in arresting embryos and consumed in healthy embryos . this might have selection potential . similarly the results for asparagine and glycine are significant . it was suspected that as more tests are carried out , the significant differences between amino acid consumption and production for healthy and arresting embryos may increase . studies were extended to human embryos from day 2 to day 3 post insemination . earlier assessment was considered advantageous since it is preferable to minimise the exposure of the developing embryo to artificial culture conditions as far as possible . the results of these experiments are illustrated in fig4 and 5 . subsequent studies involved the collection of pregnancy data . icsi embryos were analysed from day 1 to day 2 of development , before subsequent transfer into patients . the choice of icsi embryos versus ivf embryos was due to the fact that the cumulus of an icsi embryo has been removed , thus facilitating assessment . now the focus was not so much the identification of healthy , pre - implantation embryos but whether the embryo would implant within the mother ( i . e ., a move towards the clinical side ). icsi embryos were incubated in culture medium comprising ebss supplemented with 1 mm glucose , 5 mm l - lactate , 0 . 47 mm pyruvate and a physiological mixture of 20 amino acids . the individual concentrations of the amino acids were in the range 0 . 005 mm to 1 . 0 mm . the results from this line of the investigation were particularly encouraging and provide corroborative evidence for the efficacy of the assessment method . the method of the present invention allows for the generation of ‘ finger - print ’ amino acid profiles which are characteristic for a particular species and which can be used in selecting the most viable cell for that species . because it is not known which embryo will develop to term , typically two or three embryos are transferred back into the mother after artificial insemination . the method of the present invention reduces the likely of multiple pregnancy by providing a biochemical test for the selection of a single embryo . this is considerably more accurate than morphological selection methods .