56 Bashford Dean Memorial Volume 



The next figure (Number 22), apparently drawn under a greater magnification, has a 

 larger number of micromeres — eighteen or nineteen — supported by apparently only 

 twelve macromeres. In the upper right region of the micromeres are two rather un' 

 usually large intercellular spaces presumably opening into the segmentation cavity. It is 

 interesting to note that in each egg there are at least twelve vertical furrows separating 

 the macromeres, a close approach to sixteen, the theoretical number if the segmentation 

 were complete and regular. 



LATER CLEAVAGE STAGES 



Of eggs in later stages we find only two drawings, here reproduced as Figures 23 

 and 24, plate 11. In the stages represented in these figures, the discoidal character of the 

 cleavage is more clearly apparent. 



In Figure 23 the smaller micromeres are aggregated at the center of the blastoderm. 

 On the left side of this the blastomeres are larger and are also fewer than on the right 

 side and among them are what appear to be unusually large micromeres, possibly unseg' 

 mented regions of the outer edge of the blastodisc, or possibly projecting macromeres in 

 which segmentation for some unknown cause has been delayed. In the surrounding and 

 supporting mass of protoplasm, only two or three cleavage furrows are visible in the 

 drawing. 



The other egg, shown in Figure 24, differs from the preceding in that large and small 

 micromeres are mingled indiscriminately but there are lacking the unusually large cells 

 and the peninsula-like projections noted in the former. Further, almost the entire surface 

 of the blastodisc visible in polar view is segmented — i.e., there is, unlike the condition 

 in the preceding drawing, only a narrow surrounding rim of partially segmented protO' 

 plasm visible. 



The conditions in the marginal partially segmented portions of both these blastodiscs 

 agree with what Dean found in sections of a stage approximately corresponding to them. 

 Of It he writes: "No traces of cleavage could be found on the yolk region of the egg, the 

 entire series of the larger marginal blastomeres being but faintly indicated." His figure of 

 the blastoderm, reconstructed from sections of the stage just referred to, is reproduced 

 herein as Text'figure 7- This bears a marked resemblance to our latest stage shown in 

 Figure 24, plate II. 



Inspection of Dean's figures in his 1899 memoir discloses the fact that, during the 

 latest segmentation leading up to the gastrula stage, the blastoderm overgrows the whole 

 animal end of the egg, extending excentrically on the side of the future blastopore a little 

 beyond the opercular groove. It is our impression that these changes are accompanied by 

 a complete obliteration of the germinal hillock, whose progressive flattening has been 

 illustrated so far as there are drawings available. We are confirmed in this conclusion by 

 Dean's figure of a gastrula (1899 memoir, p. 252) which shows not the least trace of such a 

 hillock. 



