200 DEVELOPMENT OF FISHES 
mentation. The first plane of cleavage is a vertical one, 
passing down the side of the egg (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 egg 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, MF, 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 
