PROBLEMS OF MAMMALIAN DEVELOPMENT 



451 



ing the relationship between parent and offspring in such a type 

 of development. There are many instances of brooding and sim- 

 ilar modes of protected development in the Animal Kingdom (cf. 

 Fig. 236) ; but the example to be considered is of interest because 

 the Mammalia are the highest form of animal life. In a starfish or 

 a frog, the eggs are laid in the external water and develop indepen- 

 dently of parental influences, once the spawning is complete. Eggs 

 and sperm may even be taken 

 from the reproductive organs of 

 the parents, which are killed by 

 the operation, and when they 

 are mixed together in water nor- 

 mal fertilization and development 

 will follow. Under such a mode 

 of development, no one would 

 think of the female parent as 

 exercising an influence upon the 

 developing young other than 

 what might affect the germ cells 

 while still within the parental 

 body. The same is true of the de- 

 velopment of a bird. The parent 

 incubates the egg, but any female 

 can perform this function, and 

 chicks may often be more effect- 

 ively hatched in an incubator 

 than by hens. In the mammal, 

 however, where the young arc 



Fig. 23.5. — Human embryo of the 

 fourth month as it appears in uterus. 



r, chorion and amnion; p, placenta; u, 

 umbilical cord. (After Strahl, from Kelli- 

 retained within the parent, it is cott,""Chordate Development," copyright, 



not unnatural to think that Z'l'tLflV "°^' "'^ ^°' ""'"'"^ 



by permission.) 



changes in the mother's body 



may influence the embryo, and this is commonly supposed to be 

 the case, although there is no scientific foundation for the beUefs 

 that are usually accepted. 



For example, it is believed b}^ many persons that the foetus of 

 a human being or of any of the familiar domesticated mammals 

 may be profoundly influenced by changes in the mother during 

 pregnancy and that such influences may even be exercised by her 

 mental states. Everyone has heard stories of how the deformities 

 of certain individuals were caused by some injury to the mother or 



