290 



DEVELOPMENT OF THE ME $0 BLAST. 



certain that any conclusions can be directly drawn from them as to 



D 



en 



FIG. 208. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. 

 (From Parker ; after Keichenbach.) 



A. Section through part of the ovum during segmentation, n. nuclei; w.y. white 

 yolk; y.p. yolk pyramids ; c. central yolk mass. 



B. and C. Longitudinal sections of the gastrula stage, a. archenterou; ft. blasto- 

 pore; ms. mesoblast; cc. epiblast; en. hypoblast, distinguished from epiblast by 

 shading. 



D. Highly magnified view of anterior lip of blastopore, to shew the origin of the 

 primary mesoblast from the wall of the archenteron. p.ma. primary mesoblast ; ec. 

 epiblast; en. hypoblast. 



E. Two hypoblast cells to shew the amoeba-like absorption of yolk spheres. 

 y. yolk; . nucleus;/;, pseudopodial process. 



F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.ms.); 

 n. nucleus. 



the actual origin of the mesoblast in the triploblastic forms, till we 

 know from what diploblastic forms the triploblastica originated. 

 All that they shew is that any of the constituents of the mesoblast 

 may have originated from either of the primitive layers. 



For further light as to the origin of the mesoblast, it is necessary 

 to turn to its actual development. 



The following summary illustrates the more important modes in 

 which the mesoblast originates. 



1. It grows inwards from the lips of the blastopore as a pair of 

 bands. In these cases it may originate (a) from cells which are 

 clearly hypoblastic, (6) from cells which are clearly epiblastic, (c) 

 from cells which cannot be regarded as belonging to either layer. 



Mollusca Gasteropoda, Cephalopoda, and Lamellibranchiata. In 

 Gasteropoda and Lamellibranchiata the mesoblast sometimes originates 

 from a pair of cells at the lips of the blastopore, though very probably 



