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, bnt 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 róle 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 (792), 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, 
