SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 223 



the clitoris, a relatively small body of erectile tissue, which develops 

 ventral to both the genital and urinary apertures, is homologous to 

 the glans portion of the penis. The labia, which are folds of tissue 

 at each side, are homologous to the scrotum of the male. 



Intrauterine Development 



There are several groups of animals, particularly among chordates 

 in which the zygotes are retained within the uterus for a consider- 

 able portion of the embryonic and fetal development. In such 

 chordates as some of the common sharks ; a few of the bony fishes, 

 as Gamhusia the mosquito minnow; certain snakes and lizards; and 

 in all mammals except the lowest group, the Prototheria, there is some 

 form of intrauterine development. In the groups mentioned, other 

 than the mammals, the egg bears an abundant supply of yolk, and 

 the developing embryo, although retained in the uterus for protection, 

 depends on the yolk supplied by the egg for nourishment. In mam- 

 mals, including the human being, the egg has a very meager supply of 

 yolk. It becomes attached (implanted) in the epithelium lining the in- 

 ternal surface of the wall of the uterus, which is highly vascularized. 

 In all forms where intrauterine development occurs, as well as in many 

 others, the ovum is fertilized within the genital tract of the female by 

 introduction of spermatozoa in the act of copulation or coitus. Among 

 the mammals the penis of the male is well developed as an intromit- 

 tent organ which is received in the vagina of the female where the 

 semen, carrying spermatozoa, is discharged. The motile spermatozoa 

 swim up the oviduct and meet the ovum shortly after it enters. Fer- 

 tilization occurs here, and the zygote moves down the oviduct, de- 

 veloping as it goes. By the time it reaches the uterus the embryo 

 is in the gastrula stage of development. The embryo is attracted to 

 the wall of the uterus, where it is partially embedded and attached. 

 This procedure is knoAvn as implantation. Due to the influence of a 

 sex hormone, the uterus becomes highly vascularized and increases in 

 size. The extra-embryonic membranes develop rapidly to cover the 

 embryo. These embryonic membranes unite vdth the epithelial lining 

 of the uterus at the point of contact to form the placenta. The uterine 

 portion of this membrane receives an abundant supply of maternal 

 blood, while the embryonic portion receives a rich supply from the 

 embryo by way of the umbilical vessels which pass through the um- 

 bilical cord. In some mammals the layers of the placenta con- 

 tributed by parent and embryo become inseparably fused, but in 



