318 



CLEAVAGE (segmentation) AND BLASTULATION 



PERM 

 4 CLEAVAGE NUCLEI 



"c E N T R A L CELLS 5 P. 



B L A S T 



PERIBLAST 



Fig. 158. Early cleavage phenomena in elasmobranch fishes. (A, B, E. F, G after 

 Ziegler, '02, from Riichert; C, D after Ziegler. ) (A) Germ disc of Torpedo ocellata, 

 showing four cleavage nuclei, sperm nuclei, and beginning of first cleavage furrow. (B) 

 Stage of cleavage, possessing 16 cleavage nuclei. Four central cells and ten marginal 

 cells are evident from surface view. (C) Surface view of blastoderm of Scy Ilium 

 canicula with 64 cleavage nuclei. Twenty-nine central cells and seventeen marginal cells 

 are evident from surface view. (D) Later cleavage stage of S. canicula with 145 cells 

 showing. (E) Transverse section of (B). (F) Transverse section of blastoderm of 

 T. ocellata with 64 cells. (G) Median section through blastoderm of T. ocellata at 

 the end of the cleavage period. 



form of two layers situated immediately above the horizontal cleft or 

 segmentation cavity (fig. 156D). 



(b) The horizontal cleft or segmentation cavity gradually widens and en- 

 larges. It separates the central cells above from the uncleaved germinal 

 disc or central periblast below. 



(c) At the margins of the central cells, these cleavages may be found: ( 1 ) 

 Vertical cleavages occur which cut off more central cells from inner 

 ends of the marginal cells. As a result, there is an increase in the num- 

 ber of central cells around the periphery of the already-established cen- 



