426 BASHFORD DEAN. 
and noteworthy Polflucht occurred. Several variations are 
shown in the adjoining figure (Fig. 5), of which the most 
frequent, as in Lepidosteus, is the symmetrical meridional 
form shown at the left. The segmentation cavity takes its 
definite origin at this stage ; in the region of the animal pole 
the blastomeres are separated from the underlying yolk—the 
germ disc by a narrow fissure, which has been found to arise 
in the cleavage planes of the animal pole. ‘Thus in a section 
of the germ disc passing through the points *—* of Pl. 30, 
fig. 4, it will be seen (PI. 31, fig. 23) that the blastomere, which 
is cut nearly transversely (a), is separated below from the yolk 
region of the germ disc (yg.) ,and at the sides partly, from the 
adjacent blastomeres by the fissure-like segmentation cavity 
(sc.). And it will be further noted that the yolk region of the 
germ disc (yg.) is still in common, marginally, with the 
blastomeres. With these conditions should be contrasted the 
more shark-like features of the corresponding stage of 
Lepidosteus (Dean, op. cit., p. 17, and pl. il, fig. 26). 
Fourth Cleavage (PI. 30, fig. 5).—The plane of the fourth 
cleavage is again vertical, resembling the third cleavage in 
essential regards. Its general direction is parallel to the 
second cleavage. This stage usually results in the division of 
the germ area into blastomeres of approximately equal size ; 
often, however, as in the figure (PI. 30, fig. 5), the cleavage 
passes nearer or further from the animal pole than in the 
preceding stage: in this event the central blastomeres appear 
at the surface rectangular instead of square. Many cleavage 
variations at this stage were recorded, in which noteworthy 
Polflucht occurred, and in which meridional planes often 
replaced the transverse cleavages ; in no instance, however, 
were horizontal cleavages observed. The segmentation cavity 
in this stage does not markedly differ from that noted in the 
earlier stage. In Pl. 31, fig. 24, a section is shown passing 
through the central blastomeres in the direction of the (first or) 
second cleavage plane ; its direction is slightly oblique, and it is 
for this reason that five blastomeres appear. In this section 
the extent of the segmentation cavity may be followed, and by 
