162 THE ANCESTRY OF VERTEBRATES 



vestigation of only one or a few forms — without ever 

 considering the same processes in Invertebrates and 

 evidently regarding the germinal layers as a mere histo- 

 logical conception. 



Truly, there is often a histological or, better perhaps, a 

 cytological difference between the cells of the primary 

 entoderm and those of the primary ectoderm, resulting from 

 a difference in the amount of yolk* and well distinguished 

 from the histological difference of the tissues to which they 

 give rise in the adult form. However considerable this 

 cytological difference may sometimes be, and however early 

 in development it may become evident, it is yet only 

 of secondary significance in determining what we have 

 to call ento- and ectoderm. The first criterion in those 

 early stages is, as the names indicate: what disappears from 

 the surface by invagination ^ or delamination and what 

 remains on the outside? Thbugh in several Evertebiates 

 with determinate cleavcge we can already determine in the 

 blastula-slage with perfect certainty which cells will beCome 

 ento- and which ectoderm (and even which mesoderm), we 

 yet are not allowed by this circumstance to call the blastula 

 a gastrula, as has been done by some authors in the case 

 of Vertebrates. In the same way it is sometimes possible 

 in the gastrula to indicate in the two primary germlayers 

 the cells which will become the mesoderm. This is 

 neither a reason to deny that we have to deal in such a 

 case with a gastrula. It is only the process of the topo- 

 graphical separation by which the germinal layers originate. 

 No doubt the great cytological difference often prevailing 

 between ecto- and entoderm has developed phylogenetically 

 only after the topographical opposition, though in ontogeny 

 it often becomes apparent before the latter. 



This has often been lost sight of in determining what 

 we have to understand by gastrulation and by ecto- and 

 entoderm in Vertebrates. In the lower forms such as Acrania, 

 Cyclostomata and Amphibians the process of invagination is 

 easily recognized. The difference in the cytological character 

 of ecto- and entoderm-cells is sometimes less, sometimes 

 more, evident, though as a rule not excessive. In Amphibians it 

 is more pronounced than in Amphioxus and, as a conse- 

 quence, the process of invagination sometimes becomes 

 less evident, especially in the more yolk-laden eggs. Never- 

 theless, by means of the less yolk-laden eggs of other 



