DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 



85 



(VP), the cells (vz) are somewhat larger and more turbid, owing to 

 the yolk-granules lodged in them. The process of the formation of 

 the gastrula commences at this place. The vegetative surface begins 

 at first to be flattened, and 

 then to be pushed in toward 

 the middle of the sphere. 

 By the advance of the 

 invagination the depression 

 grows deeper and deeper, 

 while the cleavage-cavity be- 

 comes to the same degree 

 diminished in size. Finally, 

 the invaginated portion (fig. 

 ik) comes in contact with 



the inner surface of the un- 

 invaginated portion (ak) of 

 the blastula, and completely 

 the 



uk 



ik 

 ud 



Fig. 44. Gastrula of Amphioxus lanceolatus, after 

 HATSCHEK. 



ulc, Outer germ-layer ; ik, inner germ-layer ; u, 

 blastopore, or mouth of archenteron (c.d). 



obliterates 



v^ f 



cavity. As a result there has been formed out of the hollow 

 sphere with a single wall a cup-shaped germ with double walls- 

 the gastrula. 



The cavity of the gastrula, which results from the invagination and 

 is not to be confounded with the cleavage-cavity which it has sup- 

 planted, is the primitive intestine (archenteron) (ud), or the intestine- 

 body cavity (coelenteron). This opens to the outside through the 

 primitive mouth (mouth of the archenteron, blastopore) (u). 



Inasmuch as the names primitive intestine and primitive mouth 

 might easily give rise to erroneous conceptions, let it be remarked, in 

 order to preclude from the start such an event, that the cavity and 

 its external opening which arise by this first invagination are not 

 equivalent to the intestine and mouth of the adult animal. The 

 archenteron of the germ, it is true, furnishes the fundament for the 

 intestinal tube, but there are also formed out of it a number of other 

 organs, the chief of which are the subsequently formed thoracic and 

 body cavities. The future destination of the cavity will therefore be 

 better expressed by the term " codenteron." Finally, the primitive 

 mouth is only an evanescent structure among vertebrated animals ; 

 later it is closed and disappears without leaving a trace, while the 

 permanent or secondary mouth is an entirely new structure. 



The two cell-layers of the cup, which are continuous with each 

 other at the edge of the blastopore, are called the two primary 



