190 DEVELOPMENT OF FISHES 
proximately at right angles to the first; the third, which 
shortly appears, is horizontal (Fig. 202), giving rise to the 
stage of eight blastomeres; this plane, passing slightly 
above the equator, causes the upper blastomeres to be 
slightly smaller in size than those of the lower hemisphere. 
The amount of yolk in the egg, it is accordingly inferred, 
although not sufficient to prevent the passage of cleavage 
planes, is enough, nevertheless, to retard the nuclear cleav- 
ages in the region of the lower, or vegetative, pole. In 
Fig. 203, showing a vertical section of the following 
stage, another horizontal cleavage has been established in 
the upper part of the egg; the segmentation cavity is seen 
in the centre of the figure arising as the central space 
between the blastomeres. This is seen to have become 
greatly enlarged in Fig. 204, a slightly later stage where 
in vertical section is seen a greatly increased number of 
blastomeres. Repeated cleavage of all blastomeres now 
continues regularly, and results in the production of a 
blastula, a smooth-surfaced cell mass containing the seg- 
mentation cavity, SC (in section, Fig. 205); this is seen 
to be located in the region of the animal pole. In the 
next developmental stage, gastru/a, seen in section in 
Fig. 206, the primitive digestive tract, calenteron, C, is 
appearing ; it arises as an indentation of the side of the 
blastula. The ccelenteron, soon greatly increasing in depth, 
reduces in size and finally obliterates the segmentation cav- 
ity, taking the position, C, shown in section in Fig. 207. 
Here the segmentation cavity has practically disappeared ; 
the surface opening of the ccelenteron is the d/astopore, 
BP; the cell layer of the gastrula’s surface is the ecto- 
derm, EC; the cell layer lining the ccelenteron is the en-— 
toderm, EN: the coelenteron, it will be seen, is closely 
apposed to the ectoderm at the left of the figure, — the 
ee ee eee 
