19 • MAMMALIAN DEVELOPMENT 



Ihus 



hus far we have considered the theoretical aspects of development, using 

 experiments from various sources as a guide to the principles that operate 

 during growth and differentiation. In the study of the growth process we 

 found that the mammalian embryo possesses a rather unique mechanism for 

 getting its food. It is only natural, therefore, that we should ask: Does mam- 

 malian development differ basically from that of other animals? Does the 

 human egg follow the same pattern of development as the amphibian and 

 chick eggs ? Is human development governed by the same principles as those 

 which control the development of animals generally? We shall use the 

 human egg as an example of mammalian development, with some references 

 to other mammals. 



The egg contains very little yolk, and its early cleavage is similar to that 

 of the sea urchin egg (Fig. 170). The processes of gastrulation and meso- 

 derm formation, however, are similar to those of a large, yolky egg such as 

 those of reptiles and birds. A small egg with little or no yolk is therefore 

 an evolutionary consequence of the fact that in most mammals a placenta 

 replaces yolk as a source of nutrition. As a matter of fact, two of the lower 

 mammals — the duckbill and the spiny anteater — do have large yolky eggs 

 which develop without a placenta. When the placenta develops, yolk is no 

 longer necessary, but the egg of higher mammals still behaves as though a 



