250 



BIOLOGICAL LECTURES. 



whereas they are usually shown in the direction of the nuclear 

 spindles and in the earliest stages of cleavage." These obser- 

 vations, which, so far as I know, have not been duplicated in 

 the study of any other form, indicate, it seems to me, that the 

 reversal is of later date than the original spiral and has been 

 imposed upon the older form, but without removing all traces 

 of it as yet. I have sought very carefully for similar phenomena 

 in Arenicola, but without results except those mentioned above. 

 Reversals occur but rarely in Arenicola. 



3. The spiral form of cleavage ceases first in regions which 

 are to play important roles in the later development (Figs. 7, 

 10), while the regions which do not represent large areas in the 

 later stages continue to divide spirally for some generations. 

 In many of these cases it is impossible to say where the spiral 

 form of division has given place to a bilateral form, for there is 

 no sudden change, but the spiral direction simply becomes less 

 and less marked in succeeding divisions, until it is no longer 

 distinguishable as such. The cells of the third quartette of 

 ectomeres in Arenicola afford an excellent illustration of this 

 point. Conklin states that in Crepidula the bilateral divisions 

 appear very slowly and that it is difficult to determine in some 

 cases whether the divisions are really bilateral or not. This, I 

 think, will be found to be the case in many forms. It is cer- 

 tainly true of Arenicola as well as of Crepid2ila. A few cells 

 contrast sharply with the rest of the ^gg as regards the initia- 

 tion of bilateral cleavages, and these are the cells whose divi- 

 sions distribute the material for the embryo, including the 

 other cells themselves to a large extent. 



4. The probability that independent modification of the 

 bilateral form of cleavage has appeared in the different groups 

 or species is amply supported by the fact that in many cases 

 cells of similar origin have different fates, and vice versa. It 

 is sufficient here to mention a few of the more striking cases 

 of this sort. Perhaps the most striking case of all is that of 

 the annelid and moUuscan cross. In each group the cells of 

 the cross form the larger portion of the pre-trochal region, 

 though the exact relation of the various cells to the apical 

 plate is not sufficiently well known at present to afford a basis 



