GASTRULATION 



45 



be situated well within the small-celled area instead of at its hinder 

 margin. The importance of this consideration will become manifest 

 later on in connexion with the interpretation of the developmental 

 phenomena of the Amniota. 



Elasmobraxchii. — The egg of the Elasmobranch at the time 

 immediately preceding gastrulation differs from the blastula of the 

 ordinary Amphibian or Lung-fish in its much greater size. The small- 

 celled or micromeric apical portion of the blastula is represented here 

 by a relatively small mass of cells — the blastoderm — in the region of 



y.n. 



V-n. 



ffl 



F IG . 28 —Sagittal sections through Elasmobranch blastoderms (Torpedo) illustrating the • 

 process of gastrulatioD. (After Ziegler, 1902.) 



g.l, gastrular lip ; s.c, segmentation cavity ; y.n, yolk nuclei. 



the apical pole while the large-celled- portion is represented by the 

 yolk This latter is composed, practically, of a mass of yolk granules, 

 the protoplasmic matrix being reduced almost to vanishing-point. 

 As in the eggs previously described, the micromeric portion gradually 

 spreads round and encloses the yolk and here again we find the same 

 three factors at work— involution, overgrowth and delamination. 



The first step in the gastrulation process consists in the involu- 

 tion of the surface along the posterior edge of the blastoderm. This 

 involution groove spreads outwards on each side until it may ex- 

 tend along i to i the circumference of the blastoderm. The blasto- 

 derm is meanwhile spreading outwards all round and, as it does so, 

 the central part of the groove becomes deepened to form a tubular 



