84 



EMBRYOLOGY. 





CHAPTER Y. 



DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 



( GASTR^A-THEORY.) 



THE advances which are brought about during the next stages in 

 the development of the blastula depend primarily upon processes of 

 folding. By these means there arise larval forms, which are at first 

 composed of two, and afterwards of four epithelial membranes, or 

 germ-layers. 



The larval form which is composed of two germ-layers is called the 

 gastrula. It possesses an important developmental signification, 

 because, as HAECKEL has shown in his celebrated Gastrsea-Theory, 

 it is to be found in each of the six chief branches of the animal 

 kingdom, and thus furnishes a common starting-point from which 

 along diverging lines the separate animal forms may be derived. 

 As with blastulse, so in the case of the gastrula four different 

 kinds can be distinguished, according to the abundance and the 

 method of distribution of the yolk. Starting from a simple funda- 

 mental form, three further modifications have arisen, all of which, 



Ap with the exception 



of a single one which 

 is characteristic of 

 many Arthropods, 

 are to be encoun- 

 tered within the 

 phylum of Verte- 

 brates. 



The simplest and 

 most primitive form, 

 with the considera- 

 tion of which we 

 have to begin, is 

 found only in the 

 development of Am- 

 phioxus lanceolatus. 

 As has been previously shown, its blastula is composed of cylin- 

 drical cells, which are closely joined into a single-layered epithelium 

 (fig. 43). At one place, which may be designated as the vegetative pole 



dz 



VP 



Fig. 43.- Blastula of Amphioxus lanceolatus, after HATSCHEK. 

 fh, Cleavage-cavity; az, animal cells; vz, vegetative cells; 

 AP, animal pole ; VP, vegetative pole. 



