THE EVOLUTION OF MAN 



mere cylindrical embryo consisting of skin, stom- 

 ach and orifice, and swimming about freely. In 

 other cases, matters are not quite so plain, and 

 we meet with all conceivable variations. But we 

 have already seen that the biogenetic law never 

 excludes such modifications. The essential thing 

 is that even in the most daring deviations, the 

 relation to the Gastrula form is plainly percepti- 

 ble. In cases where no genuine cylinder is 

 formed, we meet at least two layers in the cell, 

 which are intended for the building of the body, 

 one of them corresponding to the intestinal mem- 

 brane of the genuine Gastrula, the other to the 

 external skin. 



Nor do these things end among vertebrates. 

 On the contrary, the ascidians as well as Am- 

 phioxus still develop a typical Gastrula, a freely 

 swimming "arch-tadpole," consisting of skin, 

 stomach and orifice. And these relations remain 

 plainly perceptible throughout the entire course 

 of things, even in the embryonic life of the higher 

 and highest vertebrates up to a man and includ- 

 ing him. We speak of the Gastrula stage also 

 [among mammals, even if the external aspect of 

 'things no longer very closely resembles the orig- 

 inal picture, but requires more careful investiga^ 

 tion to complete the proof. 



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