between the sexes already can be seen: the corpus cavernosum penis, and the corpus cavernosum of clitoris, and the corpus spongiosum penis arise from the mesodermal tissue in the phallus; they are at first dense structures, but later vascular spaces appear in them, and they gradually become cavernous. the prepuce in both sexes is formed by the growth of a solid plate of ectoderm into the superficial part of the phallus; on coronal section this plate presents the shape of a horseshoe. by the breaking down of its more centrally situated cells the plate is split into two lamellæ. thus, a cutaneous fold, the prepuce, is liberated and forms a hood over the glans. female in the female, a deep groove forms around the phallus. the sides of it grow dorsalward as the labioscrotal folds, which ultimately form the labia majora in females. the labia minora, in contrast, arise by the continued growth of the lips of the groove on the under surface of the phallus; the remainder of the phallus forms the clitoral glans. male the labioscrotal folds extend around between the pelvic portion and the anus, and form a scrotal area. during the changes associated with the descent of the testes this scrotal area is drawn out to form the scrotal sacs. the penis is developed from the phallus. as in the female, the urogenital membrane undergoes absorption, forming a channel on the under surface of the phallus; this channel extends only as far forward as the corona glandis. urogenital opening later, this opening, which is located on the dorsal side of the penis, closes from behind forward. meanwhile, the urethral plate of the glans breaks down centrally to form a median groove continuous with the primitive ostium. this groove also closes from behind forward, leaving only a small pipe running in the middle of the penis. thus, the urogenital opening is shifted forward to the end of the glans. diagram of internal differentiation diagrams to show the development of male and female generative organs from a common type a.—diagram of the primitive urogenital organs in the embryo previous to sexual distinction. 3. ureter. 4. urinary bladder. 5. urachus. cl. cloaca. cp. elevation which becomes clitoris or penis. i. lower part of the intestine. ls. fold of integument from which the labia majora or scrotum are formed. m, m. right and left müllerian ducts uniting together and running with the wolffian ducts in gc, the genital cord. ot. the genital ridge from which either the ovary or testis is formed. ug. sinus urogenitalis. w. left wolffian body. w, w. right and left wolffian ducts. b.—diagram of the female type of sexual organs. c. greater vestibular gland, and immediately above it the urethra. cc. corpus cavernosum clitoridis. dg. remains of the left wolffian duct, such as give rise to the duct of gärtner, represented by dotted lines; that of the right side is marked w. f. the abdominal opening of the left uterine tube. g. round ligament, corresponding to gubernaculum. h. situation of the hymen. i. lower part of the intestine. l. labium majus. n. labium minus. o. the left ovary. po. epoophoron. sc. corpus cavernosum urethrae. u. uterus. the uterine tube of the right side is marked m. v. vulva. va. vagina. w. scattered remains of wolffian tubes near it (paroöphoron of waldeyer). c.—diagram of the male type of sexual organs. c. bulbo-urethral gland of one side. cp. corpora cavernosa penis cut short. e. caput epididymis. g. the gubernaculum. i. lower part of the intestine. m. müllerian duct, the upper part of which remains as the hydatid of morgagni; the lower part, represented by a dotted line descending to the prostatic utricle, constitutes the occasionally existing cornu and tube of the uterus masculinus. pr. the prostate. s. scrotum. sp. corpus cavernosum urethrae. t. testis in the place of its original formation. t’, together with the dotted lines above, indicates the direction in which the testis and epididymis descend from the abdomen into the scrotum. vd. ductus deferens. vh. ductus aberrans. vs. the vesicula seminalis. w. scattered remains of the wolffian body, constituting the organ of giraldès, or the paradidymis of waldeyer.