370 WILLIAM PATTEN. 



the above view, it is plain that no invagination at the 

 posterior end of a segmented embryo can be justly 

 regarded as a gastrula; and conversely, in all segmented 

 animals the gastrula or any remnant of it must lie 

 at the very anterior end of the body. The conclusion 

 is obvious that no trace of a gastrula, any more than of 

 Annelid pre-oral lobes, is to be found in Vertebrates and the 

 higher Arthropods. 



Just as in the blastosphere the invagination of 

 endoderm to form a true gastrula is the ontogenetic 

 way of repeating what was originally a mere local 

 proliferation, so in the segmented animals the in- 

 vagination of endoderm and mesoderm to form a 

 telopore is an ontogenetic modification of a cluster 

 of proliferating cells or teloblasts. The telopore and 

 the gastrula are thus to a certain extent analogous, but in no 

 wise homologous. The formation of the telopore is com- 

 plicated by the increase in length, and by the presence of 

 both mesoderm and endoderm; but we have, I believe, in 

 Arthropods and Vertebrates a complete history of its various 

 phases. 



It is a fact of great importance that in Vertebrates and 

 Arthropods the functional endoderm exists at one time as two 

 longitudinal lateral bands — a condition, as far as I know, not 

 found in Annelids. 



The importance of yolk in modifying development has been 

 greatly exaggerated. If its presence obscures the primary 

 characters, it is extremely probable that in animals normally 

 having large eggs its absence would result in a still farther 

 modification, not in a reversion to the simpler condition. 

 Such has been the case, I believe, in Amphioxus, where the 

 simplicity in development is apparent, not real. In fact, the 

 coelom theory proves too much, for it cannot explain why Am- 

 phioxus in the development of its body-cavity falls little short 

 of being a typical Coelenterate, while the Annelids themselves 

 do not in this respect show the slightest trace of Coelenterate 

 characters. 



