112 J. Thomas Patterson. 



tralized suddenly, because sperms are found lodged in the vitelline 

 membrane, as though they had been stopped in the very act of enter- 

 ing the egg (Fig. 7). 



VII. The Four-celled Stage.- — Three and One-fourth Hours. 



The four-celled stage is produced by a vertical division in each 

 of the blastomeres of the tw^o-celled stage, and the two furrows meet 

 the first one approximately at right angles (Fig. 12). While the 

 division in one blastomere may slightly precede that of the other, 

 usually they occur simultaneously. It has been stated that the point 

 where the second furrows meet the first is situated eccentrically, the 

 displacement being toward the posterior border of the blastoderm. 

 It is not uncommon to find eggs with the center of the cleavage 

 eccentric, but the displacement may be in any direction from the 

 center. The writer does not believe, therefore, that any importance 

 can be attached to the eccentricity of cleavage. 



The rudimentary accessory cleavage makes its aj^jDearance in the 

 four-celled stage, and in the egg shown in Fig. 12 there are three 

 such furrows present. These cut across the margin of the area of 

 primary cleavage, and their planes are approximately radial. In 

 most eggs the furrows lie entirely without the margin. This blasto- 

 disc shows, in addition to the three accessory furrows, two other 

 small ones lying well within the margin, but their position in the 

 anterior blastomeres makes it clear that they are the approaching divi- 

 sions of these two cells. 



VIII. The Eight-celled Stage. — Four Hours. 

 In the formation of the eight-celled stage, the third division fur- 

 rows, at least in some cases (Fig. 16), tend to remain regular; that 

 is, each of the third furrows is vertical and meets the second furrow 

 at right angles. There is thus produced two parallel rows of four 

 cells each. In the majority of eggs, however, the form of the 

 cleavage in this stage apparently is not regular, though probably 

 if it were possible to follow the divisions in the living cells it would 

 be found that there was considerable regularity. The cells do not 

 always divide simultaneously, and since there is a tendency for the 



