78 



INVERTEBEATA 



CHAP. 



This plug which normally persists for only a short time, persists in 

 this case much longer, and eventually the cells forming the borders 

 of the area of invagination are carried in past it, so that the 

 invaginated layer, or endoderm, is represented by a solid plug of 

 material surrounded by a layer of cells. This plug is gradually 



Fig. 5S. — Four stages in the developmeut of the egg of Urticina crassicornis as seen in 

 longitudinal sections. (After Appellof. ) 



A, blastula. B, hlastula preparing to undergo invagination. C, invagination nearly complete, 

 gastrula stage. D, formation of stomo laeum 6^, blastopore ; c, remnants of the core of the egg ; eTid, 

 endoderm ; inv, area of invagination ; o, mouth ; stom, stomodaeum. 



digested by the surrounding cells and thus the hollow gastrula stage 

 is reached. 



The relation between the normal and abnormal methods of 

 reaching the two-layered stage in this species is the same as the 

 relation between the method of forming the planula larva in 

 Scypbozoa and that in Hydrozoa. We may regard them as two 

 varieties of gastrulation. The invaginated cells or endoderm. 



