282 DEVELOPMENT OF ELASMOBRANCH FISHES. 



blastoderm, the alimentary and neural canals communicate 

 posteriorly in Elasmobranchs and Osseous Fishes, but not in 

 Birds. Of all these points Professor Haeckel makes no mention. 



The support of his views which Prof. Haeckel attempts to 

 gain from Gotte's researches in Mammalia is completely cut 

 away by the recent discoveries of Van Beneden 1 and Hensen 2 . 



It thus appears that Professor Haeckel's views but ill accord 

 with the facts of vertebrate development ; but even if they were 

 to do so completely it would not in my opinion be easy to give a 

 rational explanation of them. 



Professor Haeckel states that no sharp and fast line can be 

 drawn between the types of ' unequal ' and ' discoidal ' segmenta- 

 tion 3 . In the cases of unequal segmentation he admits, as is 

 certainly the case, that the larger yolk cells (hypoblast) are 

 simply enclosed by a growth of the epiblast around them ; which 

 is to be looked on as a modification of the typical gastrula inva- 

 gination, necessitated by the large size of the yolk cells (vide 

 Professor Haeckel's paper, Taf. II. fig. 30). In these instances 

 there is no commencement of an ingrowth in the manner supposed 

 for mcroblastic ova. 



When the food-yolk becomes more bulky, and the hypoblast 

 does not completely segment, it is not easy to understand why 

 an ingrowth, which had no existence in the former case, should 

 occur ; nor where it is to come from. Such an ingrowth as is 

 supposed to exist by Professor Haeckel would, in fact, break 

 the continuity of development between meroblastic -and holo- 

 blastic ova, and thus destroy one of the most important results 

 of the Gastraea theory. 



It is quite easy to suppose, as I have done, that in the cases 

 of discoidal segmentation, the hypoblast (including the yolk) 

 becomes enclosed by the epiblast in precisely the same manner 

 as in the cases of unequal segmentation. 



But even if Professor Haeckel supposes that in the unseg- 

 mented food-yolk a fresh element is added to the ovum, it 



1 " Developpement Embryonnaire des Mammiferes," Bidletin de VAcad. r. d. 

 Belgique, 1875. 



2 Loc. cit. 



5 For an explanation of these terms, vide Prof. Haeckel's original paper or the 

 abstract in Quart. Journ. of Alicr. Science for January, 1876. 



