200 DEVELOPMENT OF FISHES 



mentation. The first plane of cleavage is a vertical one, 

 passing down the side of the ^^^ (Fig. 233) as a shallow 

 surface furrow, not appearing to entirely separate the sub- 

 stance of the blastomeres, although traversing completely 

 the lower hemisphere (Fig. 232). A second vertical furrow 

 at right angles to the first is seen from the upper pole in 

 Fig. 234; it is essentially similar to that of Fig. 233. The 

 third cleavage of Fig. 235 is again a vertical one (as in all 

 other fishes, but unlike Petromyzon), approximately meridi- 

 onal ; its furrows appear less clearly marked than of earlier 

 cleavages, and seem somewhat irregular in occurrence. The 

 fourth cleavage is horizontal above the plane of the equator. 

 Judging from Semon's figure (Fig. 236), at this stage the 

 furrows of the lower pole seem to have become fainter, if 

 not entirely lost. A blastula showing complete segmenta- 

 tion is seen in Fig. 237 ; the blastomeres of the upper 

 hemisphere are the more finely subdivided ; the conditions 

 of the segmentation cavity may be expected to prove 

 similar to those of Fig. 205. Two stages of the gastrula 

 are shown in Figs. 238 and 239, showing a full view of the 

 blastopore. In the earlier one (Fig. 238) the dorsal lip of 

 the blastopore is crescent-like ; in the later (239) the 

 blastopore acquires its oblong outline, through which the 

 yolk material is apparent ; its conditions may later be 

 compared to those of a Ganoid (Figs. 254, 255). 



The growth of the embryo is illustrated in the remaining 

 figures (Figs. 240-248). A side view of an early embryo 

 is shown in Fig. 240 ; at the top of the ^g^ to the right is 

 the head region, to the left the blastopore and tail. The 

 surface view of the head region (Fig. 241), the medullary 

 folds, MFy may be compared with those of Fig. 225, 

 although they are low and widely separated ; the axial 

 seam is referred to by Semon as a demonstration of the 



