236 



GASTEROPODA. 



In the later stages of segmentation the epiblast cells differ from the 

 hypoblast cells in the absence of pigment. The segmentation cavity, if 

 developed, is small. A perfectly regular gastrula is formed (fig. 107 A and 

 B), which is preceded by the embryo assuming a flattened form. The 

 blastopore is at first wide, but gradually narrows, and finally assumes a 

 slightly excentric position. // becomes not the mouth, but the anus. 



When the blastopore has become fairly narrow, mesoblast cells (B, me.) 

 appear around it, between the epiblast and hypoblast. Whether they are 



FlG. 107. FOUR STAGES IN THE DEVELOPMENT OF PALUU1NA V1VIPARA. 

 (Copied from Biitschli.) 



ep. epiblast ; hy. hypoblast ; me. mesoblast ; bl. blastopore ; an. anus ; st. stomo- 

 dseum ; sh. shell-gland ; V. velum; x. primitive excretory organ. 



bilaterally arranged or no is not clear; and though coloured like the 

 hypoblast, their actual development from this layer has not been followed. 



The velum appears about the same time as the mesoblast, in the form of 

 a double ring of ciliated cells at about the middle of the body (B and C, V\ 

 The mesoblast rapidly extends so as to occupy the whole space between 

 the epiblast and hypoblast, and at the same time becomes divided into two 

 layers (C). Shortly afterwards a space the body cavity appears be- 

 tween the two layers (D) which then attach themselves respectively to the 

 epiblast and hypoblast, and constitute the somatic and splanchnic layers of 

 mesoblast. The two layers remain connected by transverse strands. 



