432 WESLEY R. COE, 



not fuse, both nuclei elongate and the asters form between their ex- 

 tremities. The cleavage- spindle is theo formed, the egg-cell elongates 

 slightly, and a circular depression appears on the circumference of 

 the egg passing nearly through the point where the polar bodies were 

 given off, and at right angles to the axis of the cleavage-spindle. 

 The egg elongates still more, and the depression deepens until the 

 cell is completely divided. At a temperature of about 20*^ C the 

 first segmentation is completed in less than one hour and ten minutes 

 after fertilization ^). The resulting blastomeres are almost exactly 

 equal in size. Only occasionally was one of them slightly larger than 

 the other, so that this difference must be looked upon as accidental. 



The large aster which passes into each blastomere rapidly dis- 

 appears, and in its place a pair of small asters arise, side by side, 

 near the oval nucleus. These pass to opposite ends of the nucleus, 

 as is usual in such cases, and a cleavage spindle is formed. The 

 division into four cells, all of which are again of very nearly equal 

 size, takes place about 24 minutes after the first cleavage. The third 

 cleavage occurs thirty minutes later; the fourth after some thirty 

 five or forty minutes more, and after a further lapse of about fifty 

 minutes, or in a little less than three and one-half hours after fertiliz- 

 ation, the fifth division with its resulting 32 cells is completed. 



The segmentation of these eggs conforms to the symmetrical, 

 spiral type of cleavage, and on account of the great regularity with 

 which the cell-divisions occur they oö'er an almost ideally perfect 

 illustration of this type. The cells are so exceedingly clear and their 

 outlines so sharp that they would be admirably adapted for the study 

 of the cell-lineage of the embryo were it not for the fact that in the 

 earlier stages they are all of so nearly the same size that the orient- 

 ation of the egg is rendered very difficult. Nevertheless the cells at 

 the two poles show peculiarities similar to those found in the Annelid 

 and Polyclade eggs, so that it is reasonable to hope that by careful 

 and extended study all the details of the cell-lineage may be 

 worked out. 



Gastrulation begins about seven and one -half hours after the 

 eggs are fertilized, and in one and one-half hours more the embryos 

 begin to swim. 



1) If the eggs are allowed to remain in the water until the first 

 polar spindle has formed before they are fertilized, the two-celled stage 

 results in about 55 minutes after fertilization. 



