Emhrt/ology of certain of the Loiver Fishes. 193 



the main part (A) of the gastrulation process consists of the 

 tucking in of the hypoblastic part of the blastula wall ; it is 

 only in its latest stages (B) that it consists in an onward move- 

 ment of the blastopore lip. In Polypteriis and Amphibians, 

 owing to the enormous thickening of the hypoblast due 

 to its storage of yolk, the process A is rendered mechanically 

 impossible until in the latest stage of gastrulation, when the 

 blastopore lip has already been carried almost completely 

 over the mass of yolk. In the stage in Polypterus of which 

 I have been speaking, we see in the deep groove round the 

 huge yolk-plug the attempt on the part of the Qgg to carry 

 out the gastrulation process in the ancestral way. 



Lepidosiren. 



The method of gastrulation is shown in my previous 

 papers.^ The process begins with the appearance of a 

 shallow latitudinal groove running round about one-third of 

 the circumference of the egg at its latitude. This groove 

 gradually shortens up, its terminal parts flattening out. It 

 is only in its central portion that the groove deepens and 

 gives rise to the archenteron of the gastrula. In PolyiJterus 

 we have seen that the invagination groove still becomes a 

 closed curve at an early stage in gastrulation, and in the 

 Amphibian usually only at a much later stage. In Lepidosiren 

 the curve never becomes closed at all, its terminal parts 

 disappearing. Finally, in the Elasmobranch, the only part 

 of the invagination curve which ever appears is the central 

 part, which gives rise to the archenteron. 



Primitive Arrangement of the Cell Layers. 



There do not appear to exist in the data of vertebrate 

 embryology, as known to us at present, any distinct traces 

 of the descent of this phylum from any other phylum of 

 triploblastic Metazoa, though there are various morphological 

 considerations which make it seem probable that vertebrates, 



1 Phil. Trans. Hoy. Soc, vol. 192, B; and Quart. Jour. Micros. Sci., 

 vol. 45. 



