48 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



portion of the yolk " noted by Mark ('81), there being no evidence of a 

 well differentiated vitelline membrane. If the latter, then there is 

 physical continuity in the living substance of the two blastomeres, and 

 the appearance in the living egg is deceptive. Experimentation might 

 possibly settle this interesting question. 



The two cells now lose their individual spherical contours, owing to 

 their apposition and the mutual flattening of the two faces in contact. 

 Thus the nearly spherical outline of the whole egg is re-established. See 

 Figures 1-6, which form a series showing successive conditions of a single 

 egg, and render a detailed description of the process unnecessary. 



The alternation of the rounded and flattened condition of cells during 

 and subsequent to mitosis has been very generally observed in the stages 

 of cleavage, but the explanation of the causes which underlie this 

 phenomenon seems as remote as ever. Whitman ('78), in his discussion 

 of the cleavage of Clepsine, concludes : " The cause of the separation 

 and of the subsequent approach is undoubtedly the nucleus. . . . The 

 proof that this is an electrical phenomenon is at present wanting, but the 

 facts seem to point in this direction very strongly." Our increased 

 knowledge of the part that the cytoplasm plays in the process of cell 

 division, especially the directive role of the centrosomes, has suggested 

 another influence to which we may turn for a solution, though the nature 

 and exact operation of that influence is by no means definitely settled. 



This first cleavage plane divides the egg into equal or approximately 

 equal cells. In some cases, by careful measurement, a slight difference 

 in size could be detected ; in one case, for example, one of the cells 

 measured 19 X 26 units of the ocular micrometer, and the other 20 X 27, 

 when viewed from the animal pole. The theoretical consideration of the 

 orientation of the early stages will be taken up later; suffice it for the 

 present to say that the orientation adopted by Wilson ('92), and later by 

 Conklin ('92) and Heymons ('93), will be employed in the present paper. 

 The first cleavage plane, then, cuts the egg into an anterior half, A B, 

 and a posterior half, C D, i. e. it is perpendicular to the antero-posterior 

 axis of the egg. 



The discussion of the cleavage cavity will also be deferred till a later 

 part of the paper is reached. 



Third Generation. Second Cleavage Furrow. Four Cells. 



Plate I. Figs. 6-8; Plate II. Fig. 15-18. 



The second furrow is formed, at the ordinary temperature of the labo- 

 ratory, about two hours after the appearance of the first. Like the first, 



