REPRODUCTION 



535 



the posterior gut and forces its way into 

 the extraembryonic cavity to become a 

 large sac-Hke structure, the outer layer of 

 which fuses with the chorion. The resulting 

 intimately fused membrane then lies in 

 close proximity with the inner surface of 

 the shell in birds and reptiles but in mam- 

 mals eventually becomes a part of the pla- 

 centa. The allantois functions as a respira- 

 tory organ in picking up oxygen and giving 

 off carbon dioxide for the developing em- 

 bryo; in addition, it absorbs food materials 

 from the large egg of birds and reptiles and 

 acts as a repository for nitrogenous wastes. 

 In higher mammals the allantois, to- 

 gether with the chorion, comes into tem- 

 porary contact with the uterine wall where 

 the chorion sends out finger-like projec- 

 tions, the chorionic villi, deep into the wall's 

 soft tissues. This region of contact is richly 

 supplied with capillaries from the umbilical 

 artery. Embryonic blood is sent to this re- 

 gion under the impetus of the fetal heart- 

 beat and returned to tlie embryo via the 

 umbilical vein. Simultaneous with the de- 

 velopment of the chorionic villi, the uterine 

 wall in the same region becomes highly 

 vascularized, forming many blood spaces 

 into which the finger-like villi dip. This 

 entire region is known as the placenta, 

 which is shed at birth. It must be remem- 

 bered that there is no blood connection be- 

 tween the embryo and the mother; each has 

 its own circulation which is kept distinct at 

 all times. The placenta acts like the attach- 

 ment organ of a fungal parasite — a means 

 of extracting nourishing fluids from the 

 "host." Through this organ oxygen is ob- 

 tained, carbon dioxide is eliminated, food is 

 absorbed, and nitrogenous wastes (urea) 

 are discharged. 



Evolution of external structures 



We usually think of the need for external 

 genitalia to facilitate the union of the sex 

 cells as directly related to the change to 

 life on land. To be sure, such accessory 

 structures were essential for land life, but 



we must not forget that copulatory organs 

 did evolve among fishes that never left the 

 water. The sharks again are a striking ex- 

 ample. The claspers, which are modified 

 pelvic fins in the male shark, are utilized as 

 an intromittant organ for carrying the sperm 

 to the cloaca of the female (Fig. 21-2). 

 Many bony fishes also have converted their 

 fins into a copulatory organ. These are un- 

 usual cases among the fishes and most of 

 them have so little external sexual dimor- 

 phism that even the best ichthyologists 

 have difficulty in telling male from female. 

 This is no problem for the fish, however! 



Among the strict land vertebrates — rep- 

 tiles, birds, and mammals — a special copu- 

 latory organ, the penis, has evolved. Among 

 the reptiles it is a pair of elongated masses 

 of erectile tissue with a groove between, 

 the entire structure originating from the 

 floor of the cloaca. In mammals the groove 

 is closed to form a tube which is a direct 

 continuation of the urethra. The terminal 

 portion of the female tract has followed a 

 parallel evolution by becoming transformed 

 into a tube-like receptacle, the vagina, for 

 the penis and seminal fluid. 



Along with these changes has come the 

 introduction of a large complex of chemical 

 regulators (hormones) and certain nervous 

 modifications which has made copulation 

 not only a necessity for the continuance of 

 the race but a highly gratifying experience. 

 During the breeding period of large mam- 

 mals such as cattle both sexes will go to 

 great lengths, even exposure to death it- 

 self, in order to tend to the business of 

 bringing about the union of their sex cells. 

 Owners of female dogs are well aware of the 

 semiannual heat cycle of their charges. This 

 intensity of the sex drive in animals is im- 

 perative for race survival. One can imagine 

 how long any race would last if it were 

 lethargic in this respect. It is easy to under- 

 stand how those animals with the greatest 

 sex drive were more apt to become the 

 parents of the next generation, whereas 

 those indifferent in this regard might never 



