388 



PROGRESSIVE DEVELOPMENT 



mals ; in man, it is a vestige. The muscles of the ears of human 

 beings are quite functionless although they are of value in aiding 

 lower animals to hear. 



Embryological evidence. All animal embryos go through the 



IsVz sctlavncmJler* -tortoise Clrvictc 



Embryos of various animals appear to go through very similar stages. Before differentia- 

 tion has progressed very far, it would be very difficult to recognize a particular embryo. In the 

 later stages of embryonic development, species take on special characteristics. 



blastula and gastrula stages. The most primitive many-celled 

 animals we know are the sponges. They are simple gastrulae. 



The embryos of many vertebrates show similarities that are 

 not noticed in the fully developed organisms. They all go 

 through a similar early development. The nearer alike the adult 

 organisms are the longer they will show similarities in their de- 

 velopment. The higher vertebrates, the mammals, in their early 

 stage of development, have gill openings similar to those found in 

 fish. During embryological development, the diaphragm, which 

 in the fishes shuts off the gill chamber from the rest of the body, 

 in the human embryo moves well down into the body cavity. 



While the human embryo is at one stage in its development, it 

 has a tail-like structure. The generalization of the development 



