ORIGIN OF THE GERMINAL LAYERS. 



351 



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. 



en 



FIG. 208. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. 

 (From Parker; after Reichenbach. ) 



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

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



B. and C. Longitudinal sections of the gastrula stage, a. archenteron; b. blasto- 

 pore; ms. mesoblast; ec. 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.ms. primary mesoblast ; ec. 

 epiblast ; en. hypoblast. 



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

 v. yolk ; ;/. nucleus ; /. pseudopodial process. 



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

 n. nucleus. 



I. 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, (b) 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 



