DEVELOPMENT OF SHARK 



195 



cleavages. A stage in which early horizontal cleavages 

 are represented is shown in Fig. 220. This may well be 

 compared with the last figure ; the germ disc, while not 

 increasing in diameter, is now seen to have multiplied its 

 blastomeres by horizontal cleavages ; it is converted into 

 a plug-shaped mass of cells, sunken into the yolk material. 

 At M' are cell nuclei, which have found their way into the 

 adjacent yolk, and which there acquire a developmental 

 importance. They become the so-called merocytes, or 

 yolk nuclei. 



The section of the germ shown in Fig. 221 represents 

 a subsequent stage of development ; the blastomeres, by 

 continued subdivision, have become greatly reduced in size, 

 and are clearly to be distinguished from the smooth-sur- 

 faced, yolk-like material lying beneath. Merocytes, M', 

 are apparent in the superficial layer of the yolk ; they are 

 supposed to serve a twofold function, on the one hand, to 

 elaborate the yolk material and fit it for the embryo's use ; 

 on the other, to supply the cells which are being con- 

 tinually added to the germ's margin. In the figure a large 

 cavity is shown to exist between the yolk and the mass of 

 blastomeres. This cavity has been identified as the seg- 

 mentation cavity, SC, and the developmental stage as the 

 blastula ; it is as though the lower hemisphere of the 

 lamprey's blastula (Fig. 205) had become enormously 

 enlarged, and all traces of the cells in the floor of its 

 segmentation cavity lost, except in the layer of the 

 metamorphosed cells, the merocytes. 



In the next growth process the extent of the germ area 

 becomes greatly increased ; the thick blastula is now 

 thinned out into a surface layer of regular cells, an en- 

 larging disc-like blastoderm, which will eventually grow 

 around and enclose the entire egg. The blastoderm of 



