286 



THE EVOLUTION OF 3IAN. 



germ or embryo, but as essentially a part of the latter, a 

 part of its intestinal tube. According to this view, the 

 primitive intestine (jyrotogaster) of the Gastrula of the 

 higher animals has separated, in consequence of the keno- 

 genetic formation of the nutritive yelk, into two different 

 parts : the after-intestine (metagaster) or the permanent 

 intestinal canal, and the yelk-sac or navel-vesicle. 



If the germ-histories of the Amphioxus, the Frog, the 

 Chick, and the Kabbit are comparatively studied (Plates II., 

 III.), I am convinced that there can be no doubt as to the 

 correctness of this view, which I have thus explained. In 

 the light afforded by the Gastrsea Theory we shall regard 

 the structural proportions of the Amphioxus alone of all 

 Vertebrates, as original and but slightly varied from the 

 palingenetic germ-forms. In the Frog these proportions are 

 on the whole but slightly kenogenetically altered. In the 

 Chick, on the contrary, they are very much altered, and 

 most of all in the Rabbit. Both in the Bell-gastrula of the 

 Amphioxus and in the Hood-gastrula of the Frog, the germ- 

 layers are visible from the first in the form of closed tubes 

 (Plate II. Fig. 6, 11). But, on the other hand, the em- 

 bryonic Chick (in the freshly-laid, unincubated egg) appears 

 in the form of a flat, circular disc ; it was only quite 

 recently that the true gastrula-character of this germ-disc 

 was recognized by Rauber and Goette.'^^ This Disc-gastrula 

 grows and surrounds the huge globular yelk, and the after- 

 intestine (metagaster) parts off from the external yelk-sac; 

 in these two processes all that is diagrammatically repre- 

 sented in Fig. 70 is accomplished ; and these are the pro- 

 cesses which have been regarded as main acts, though they 

 are in reality only secondar}'- acts. 



