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 ^^%, 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 ^^^ ; 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 

 blastulay 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, gastrnla, seen in section in 

 Fig. 206, the primitive digestive tract, coelenteron, C, is 

 appearing ; it arises as an indentation of the side of the 

 blastula. The coelenteron, 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 coelenteron is the blastopore, 

 BP\ the cell layer of the gastrula's surface is the ecto- 

 derm, EC; the cell layer lining the coelenteron is the en- 

 toderm, EN: the coelenteron, it will be seen, is closely 

 apposed to the ectoderm at the left of the figure, — the 



