Segmentation of the Egg of Bdellostoma ' 55 



The conditions shown in Figure 18, plate II, are probably abnormal. We here find 

 at least eight blastomeres of which only one is completely separated from its neighbors. 

 The chief peculiarity, however, of the egg shown in this figure is the presence of a large 

 open segmentation cavity. This may possibly be a development of the conditions shown 

 in Figures 8 and 9, plate I; or it may be due to the death and disappearance of a central 

 micromere. In this matter, we find in the brief notes appended to Dean's list of "Seg' 

 mentation Stages" a reference to an egg as follows "seven c[ells], one central lost." This 

 is the only figure we find approximating seven cells, and it is the only one in which we find 

 a blastomere missing. Morever, Dean (1899 memoir) notes that occasionally a blastomere 

 is torn away from a blastoderm in the process of removing the egg-shell. 



FOURTH CLEAVAGE STAGE 



Beginning with this stage we find in all the drawings left by Dean marked irregulari- 

 ties in the cleavage pattern. Of these drawings we have selected for description and 

 publication those which seem to us most typical. The earliest of these, which shows the 

 beginning of this stage, is seen in Figure 19, plate II. In this blastodisc some of the fourth 

 'cleavage furrows are horizontal or circular, cutting off five irregularly shaped micromeres of 

 unequal size; the remaining fourth cleavage furrows are vertical. It is notable that the 

 radiating furrows have not extended very far from the center of the germinal hillock. This 

 retardation we interpret as due to the fact that the outlying protoplasm of the blastodisc 

 is considerably laden with yolk. 



Similar in the stage of its development to the blastoderm just described is the one 

 shown in Text-figure 6 drawn from a reconstruction of a sectioned blastodisc and pub- 

 lished by Dean in his 1899 memoir. This blastoderm consists of four micromeres of mod- 

 erate and nearly equal size, of two very small ones, and of nine macromeres partially 

 separated from each other. Such a cleavage pattern might readily arise by further develop- 

 ment of such a blastodisc as that portrayed in Figure 15, plate II. It is evident that 

 some, at least, of the fourth cleavage planes are circularly arranged. 



The drawing that seems to us to show the greatest regularity in the fourth cleavage 

 furrows is the one reproduced as Figure 20 of plate II. The entire group of 14 micromeres 

 is cut off from the surrounding and underlying macromeres by circular or horizontal fur- 

 rows which belong presumably to the fourth generation. Within this mass of micromeres 

 some divisions have occurred by means of the fifth cleavage furrows which are variously 

 circular, diagonal, or vertical. It is noticeable that the micromeres vary considerably 

 in size, and that development is excentric since the micromeres extend farther from the 

 animal pole on one side of the germinal hillock. 



FIFTH CLEAVAGE STAGE 



This stage might be expected to have sixteen micromeres. We find drawings of two 

 eggs which we determine as approximately of this stage. The first of these is reproduced 

 as Figure 21, plate II. Here are shown twelve micromeres surrounded by about as many 

 macromeres incompletely separated from each other. 



