232 bulletin: museum of compaeative zoology. 



Let us examine still anothei* series of drawings (Figs. 35-42, Plates 

 VI. and VII.) made from the living egg, which in this case is viewed 

 from the ventral side and a little obliquely. The polar globules of 

 course are not seen, since they lie on the opposite side of the egg. 

 Neither is the point of view a favorable one to bring the posterior polar 

 regions clearly into profile as in the series last examined. 



Figure 35 (Plate VI.) shows the 4-cell stage ; Figures 36 and 37, 

 successive views of the 8-cell stage; and Figure 38, a 12-cell stage, the 

 four cells of the ventral hemisphere having divided in this case a little 

 earlier than those of the dorsal hemisphere. This is unusual, for the 

 difference in rate of cleavage of the cells of the two hemispheres com- 

 monly first appears, as we have seen in the three series previously 

 examined, in passing from the 16-cell stage to one of 24 cells. 



Figure 39 (Plate VII.) gives a view of the egg five minutes after the 

 stage shown iu Figure 38 had been reached. It represents the 16-cell 

 stage. A drawing made five minutes later still is shown in Figure 40, 

 and one made ten minutes after that is shown in Figure 41. 



In the last mentioned figure, the cells of this uppermost hemisphere 

 are seen to have again become rounded in outline preparatory to the 

 next cell division. Spindles are already visible in them, as indicated by 

 the arrows, those last to appear being the ones in the small cells 

 ( C^-^, D^-^) at the lower mai'gin of the figure. The subsequent division 

 was about a minute later in these two cells than in the others of the 

 same hemisphere ; this is regularly the case in the cell division which 

 leads to the 24-cell stage. 



Figure 42, the last of the series, will be at once recognizeil, by one 

 who has read my preliminary paper, as a ventral view of the 24-cell 

 stage. (Cf. Plate IX. Fig. 51.) The posterior end is clearly marked by 

 the small cells C"-^, D^-^. A re-examination of Figures 36 and 37 

 (Plate VI.) shows that the rule previously stated for orienting the egg 

 at the 8-cell stage is again exemplified in the case of this series, for in 



tlie existence during karyokinesis of astral fibres whicli attach to the cell wall at 

 particular points and by their contraction depress its surface. 



Such an explanation seems to me inadequate, at least for this case ; first, because 

 I have seen no evidence of the existence of astral fibres in karyokinesis ; secondly, 

 because at successive cleavages the prominences appear in the same structurally 

 peculiar region, whether the karyokinetic spindle is directed toward that region — 

 as the explanation of Van Beneden et Julin would imply — or not (see Plate VIIL 

 Fig. 47) ; thirdly, because astral fibres, if present, should appear in every blastomere 

 at karyokinesis, but I have been able to discover these peculiar prominences only 

 in the particular regions already described. 



