534 
in the living egg. One observes the formation of numerous verticals 
and meridionals, while the examination of serial sections shows the 
continued formation of horizontals, until the blastodisc is now about 
three rows of cells deep. It is to be specially noted that the marginal 
grooves terminate in the region of the equator. Concerning this 
cleavage DEAN says: “In the sixth cleavage the only cell divisions 
that were noted as generally constant were those of the marginal 
cells: these undergo meridional cleavage, similar to the former one, 
its furrows extend no further than the margin of the cell-cap”. 
Fig. 4. 5 hrs. after fertilization. Fig. 5. 6 hrs, after fertilization. 
Fig. 5 shows the extent of the superficial cleavage 6 hrs. after 
fertilization. Irregular cleavage now is going on in places over the 
surface of the egg, the rhythmical cleavage of the earlier stages 
being lost in the constant growth of the egg. The marginal grooves 
have increased in number; one or two of these appear to be deeper 
and longer than the remainder, as indicated in the figure. It is 
presumed that these represent the earlier grooves. None of them, 
however, in this or later stages are ever observed to extend much 
below the zone of the equator. Vertical sections of the egg show the 
blastodisc to be four or five rows of cells deep and forming the roof 
of a more or less distinct cleavage cavity. Horizontal sections along 
the equator show that none of the furrows, at this level, have cut 
deeply into the yolk; even at a level midway between the equator and 
the margin of the blastodisc no furrows have been found which had 
cut the center of the egg, the greater portion extending no more than 
half the distance from the periphery to the center. 
Sections of the egg of Lepidosteus reveal a most peculiar blastodisc 
in that it closely resembles that of Amia plus a conical elongation 
