Segmentatioji of the Egg of Bdellostoma 53 



presumably leads into the beginning of a segmentation cavity. In Figure 9 the distinctive 

 features already noted in Figure 8 are still more marked. Here the most striking chat' 

 acteristic is the rounding up of the individual blastomeres resulting in the appearance of 

 constriction from the underlying protoplasm and also in the formation of an unusually large 

 segmentation cavity. 



Thus far we have been dealing with eggs in which the cleavage pattern is very 

 regular. However, beginning with the second cleavage stage irregularities occur as 

 shown in figures now to be considered. The slightest irregularity which Dean portrays 

 is that shown in Figure 10, where the cleavage furrows are sinuous in the region of inter' 

 section. At the intersection is a slight depression of the surface of the germinal hillock. 



The irregularity of the cleavage pattern in the eggs represented in Figures 11 and 12 

 is in our opinion the result of the failure of the second cleavage furrows to meet each other 

 where they join the first. In their initial formation they evidently met the first furrow at 

 right angles but a shifting of furrows occurred to enable the blastomeres to assume a more 

 compact form. This interpretation is based on the study of the living eggs of other verte' 

 brates as well as on the examination of these figures. 



That such a slightly irregular cleavage pattern is not uncommon in the egg of Bdel' 

 lostoma is evidenced by the fact that we have another example in Text-figure 4 reproduced 

 from Dean's drawing of a wax'plate reconstruction from sections of another egg. This 

 figure practically duplicates numbers 11 and 12 of Plate I. 



THIRD CLEAVAGE STAGE 



The third cleavage furrows are usOally vertical and at right angles to one of the 

 preceding furrows, ordinarily the second. In Plate II, figure 13, is shown in lateral view 

 the beginning of the third division which will result in the formation of eight blastomeres. 

 The third furrows on the side of the egg next the observer are plainly shown but the corres' 

 ponding ones on the further side are not visible. Figure 14 on the same plate shows in 

 polar view an egg in the same stage of cleavage. This drawing was possibly made from the 

 same egg as Figure 13 of plate II, since on the side of the figures toward the observer the 

 furrows are identical. The irregularities in the egg shown in Figure 14 are due to the fact 

 that the third cleavage furrows shown in the upper part of the figure extend horizontally 

 or circularly whereas the others are vertical. In the upper left hand part of this figure 

 there is shown the beginning of a vertical cleavage furrow which probably belongs to the 

 fourth generation. 



Figure 15 on plate II shows a still more irregular cleavage pattern, which seems to us 

 to be derived from an earlier condition of irregular cleavage such as is shown in Figures 

 11 and 12 of plate'I. In all three figures the first and second cleavage furrows meet to form 

 obtuse angles, but we cannot certainly distinguish the second from the first. In Figure 

 15, plate II, we find cleavage furrows extending horizontally cutting off two complete 

 micromeres near the center of the blastodisc; these furrows are presumably third and 

 fourth cleavage furrows but they cannot be identified with certainty. The blastoderms 



