228 



THE CONTINUITY OF THE RACE 



birds and reptiles the yolk forms all the true egg except a very small polar 

 cap of cytoplasm, and the egg is supplied with additional food material, 

 the "white," which is not truly a part of the egg but a secretion formed by 

 the oviduct. In the frog the egg hatches into a larval form, a tadpole, while 

 the better supplied fish, reptile, and bird eggs hatch into baby animals 

 quite like the parents in all but size and minor features. 



In the case of mammals and a few other animals the egg is not laid 

 but is retained in the body of trie mother, where it develops; the offspring 

 is "born" with the embryonic stages already completed. This habit is 

 termed viviparous. The mammal egg contains very little yolk, just 



Fig. 15.9. Parental care. The obstetrical toad, Alytes, of Europe. The male carries the fer- 

 tilized eggs until they hatch. (Courtesy American Museum of Natural History.) 



enough to carry it through the first few stages of development. The 

 early embryo very quickly develops a circulatory system and a large 

 area of extraembryonic membrane, the placenta, which grows into very 

 intimate contact with the mother's uterine wall. Through the placenta 

 the embryo receives its needed supplies and rids itself of the by-products 

 of metabolism. 



A few organisms show a reproductive habit that appears somewhat 

 intermediate between an oviparous and a viviparous habit. In some 

 of the snakes, including the rattlesnake and its kin, and in some of the 

 insects, an oviparous type of egg is formed that instead of being laid, 

 is retained in the mother's reproductive tract until it hatches, although 

 the embryo is "insulated" from any functional contact with the mother's 

 tissues by the eggshell. This is essentially an oviparous habit but has 



