104 



INTRODUCTION. 



ferentiations of the two primary layers, and in all groups with a well- 

 developed body-cavity it is divided into two strata. One of them forms 



part of the body- wall and is known as the somatic 

 mesoblast, the other forms part of the wall of 

 the viscera and is known as the splanchnic 

 mesoblast. 



A very large number not to say the majority 

 of organs are derived from parts of two of the 

 germinal layers. Many glands for instance have a 

 lining of hypoblast which is coated by a meso- 

 blastic layer. 



The processes by which the germinal layers 

 take their origin are largely influenced by the 

 character of the segmentation, which, as was shewn 

 in the last chapter, is mainly dependent on the 

 distribution of the food-yolk. When the segmen- 

 tation is regular, and results in the formation of 

 a blastosphere, the epiblast and hypoblast are 

 usually differentiated from the uniform cells form- 

 ing the wall of the blastosphere in one of the two following ways. 



(1). One half of the blastosphere may be pushed in towards the 

 other half. A two-layered hemisphere is thus established which soon 

 elongates, while its opening narrows to a small pore (fig. 55). The 



FIG. 55. DIAGRAM 

 OF A GASTRULA. (From 

 Gegenbaur.) 



a. blastopore ; b. 

 archenteron; c. hypo- 

 blast ; d. epiblast. 



Fig.l 



Tig. 2, 



FIG. 56. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY DELAMINATION. (From 



Lankester. ) 

 Fig. 1. Ovum. 



Fig. 2. Stage in segmentation. 



Fig. 3. Commencement of delamination after the appearance of a central cavity. 

 Fig. 4. Delamination completed, mouth forming at M. 

 In fig. 1, 2 and 3 EC. is ectoplasm, and En. is entoplasm. 

 In fig. 4 EC. is epiblast and En. hypoblast. 



