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 J’ 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, 1/7’, 
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 d/astoderm, which will eventually grow 
_ around and enclose the entire egg. The blastoderm of 
