30 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



phenomenon the accessory furrows flattening out and disappearing 

 as the accessory sperm-nuclei degenerate. 



Again as in the Elasmobranch a yolk-syncytium is developed 

 beneath and around the segmented portion of the blastoderm. 



A marked difference between the Sauropsidan 1 and the Elasmo- 

 branch type of egg at a fairly advanced stage of segmentation 

 becomes apparent on comparing them with the corresponding stages 

 of eggs of a less markedly telolecithal character [e.g. Fig. 14). It is 

 seen that the blastoderm in an Elasmobranch such as that shown in 

 Fig. 8 D, E corresponds to the mass of micromeres of the holoblastic 

 egg, while in the Sauropsidan it corresponds to the mass of micromeres 

 together with the apical ends of the large macromeres. 



This is really an expression of the fact that in the Sauropsidan 

 the germinal disc extends outwards into the main yolk, and shades 

 off gradually into it. The result is that the segmentation process in 

 the outer portion of the blastoderm is delayed by the presence of 

 yolk precisely in the same way as in the lower portion of the holo- 

 blastic egg. 



r GASTRULATION 



The segmentation process is in the more primitive Vertebrates, as 

 in many other groups of the Metazoa, succeeded by a process of 

 gastrulation, in which the blastula becomes converted into a gastrula 

 i.e. a type of embryo consisting of the two primary cell-layers, 

 ectoderm and endoderm, enclosing a cavity, the archenteron, which 

 corresponds morphologically with the coelenteron of the Coelenterate 

 and which opens freely to the exterior. 



While the process of gastrulation is fairly clear in the most 

 primitive vertebrates it becomes less and less so in the more highly 

 modified members of the group until finally in the Amniota it 

 becomes completely obscured. To facilitate the understanding of 

 the modifications which the process of gastrulation undergoes it will 

 be well to study it first as it occurs in three of the more primitive 

 Vertebrates namely Amphioxus, Polypterus, and Lepido siren. 



(1) Amphioxus. — The blastula of Amphioxus is composed of a 

 single layer of cells, those towards the apical pole being smaller, 

 those on the opposite side being larger and containing in their cyto- 

 plasm larger and more numerous granules of yolk. The process of 

 gastrulation is ushered in by the large-celled portion of the blastula- 

 wall becoming flattened as shown in Fig. 18 A. The portion of the 

 flattened area which, as shown by later stages, is anterior in position 

 develops a slight depression (Fig. 18, B) which gradually deepens 

 and at the same time spreads backwards (Fig. 18, C, D, E, F, G) until 

 the large-celled portion of the embryo is completely invaginated 

 within the small-celled portion and the two-layered gastrula stage is 

 attained. 



1 See Chapter X., Segmentation of Fowl's egg, F, G, H. 



