508 CELL DIVISION IN EGGS OF CREPIDULA. 



II. Cleavage of Isolated Blastomeres. 

 (Plate XLV. Exps. 855-857, 887-892, 921-931, 957-959.) 



The cleavage in Crepidula belongs to that type which I once (1897) called 

 "determinate," in which every blastomere has a definite prospective significance, 

 giving rise to a definite part of the embryo and larva. Is this " determinism of 

 development" due to intrinsic causes, to the character of the ooplasm in a 

 blastomere, or to environmental influences acting on the blastomere, such, e. g., 

 as the interrelation between different blastomeres, etc.? Is the "potency" of 

 a blastomere of the Crepidula egg as limited as is its "prospective significance"? 

 This is a problem to which much attention has been devoted in the case of the 

 development of other animals, but although most of my observations on this 

 subject in the case of Crepidula were made many years ago, nothing has hitherto 

 been published regarding it. It is undesirable at this place to review the very 

 extensive literature on this subject, which Korschelt and Heider call Das Deter- 

 minationsproblem, and for an excellent discussion of the work on this problem 

 prior to 1902, reference is made to Korschelt und Heider, Lehrbuch der vergleich- 

 enden Entwicklungsgeschichte, Allgemeiner Theil, II Capital. 



In general, the blastomeres of the egg of Crepidula are closely adherent to 

 one another so that it is difficult to separate them without injury; nevertheless 

 in the anaphase of the first and second cleavages the blastomeres become so 

 rounded that they touch one another only by relatively small surfaces and at 

 this stage they may sometimes be separated by pressure or by shaking, without 

 injuring the cells. Sometimes such isolation is brought about by diluted sea 

 water, or by ether, but the injury to the cells in such cases is usually considerable 

 so that the subsequent development of such isolated blastomeres is complicated 

 by such injury. In an egg with such a definite cleavage pattern as that of 

 Crepidula, where the position, size and histological character of many cells are 

 highly characteristic, the study of the cleavage of isolated blastomeres is of more 

 than ordinary interest. It is here possible to determine with certainty whether 

 the cleavage goes forward in strictly partial fashion, whether it undergoes certain 

 modifications although still remaining partial in other respects, or whether the 

 blastomeres return to the condition of the unsegmented egg after their isolation. 



In practically every instance it is found that the isolated blastomeres round 

 up and in subsequent cleavages the cells come into contact with one another 

 forming contact surfaces which are often different from those of the normal egg; 

 this flattening of cells against one another usually involves slight rotation of the 

 cells and a consequent slight shifting of the cell axes with reference to the original 

 cell complex. But with this exception the cleavage of isolated blastomeres is 

 strictly "partial." The general direction of cleavages, and the size and histo- 

 logical character of the blastomeres are in all cases like those of the normal egg. 

 If isolation takes place in the 2-cell stage each ^ blastomere divides equally, 



