50 bulletin: museum of compakative zoology. 



in the short axes of the cells, as shown in Figure 75 (Plate 9). Just 

 before division the cells elongate until the axis in which the spindle lies 

 is longest, as shown in the cell 6^-^- of the figure last mentioned. 



The cells a'-^^-c'-^^, near the animal pole, cleave meridionally. The 

 spindle in a'•l^ and the cells b^-'^ and c^-^^ b^-^'> and c^-^", are shown in 

 Figure 72 (Plate 9). The minute cells a^-^-c^'-^*', like d'-'^^, do not 

 cleave further. The arrangement at the animal pole is now very 

 irro'ndar, and owing to this fact a)id the minuteness of the cells pro- 

 duced at the last cleavages it is very difficult to be certain of the exact 

 ori"in of any given cell, though the origin of the group cannot of 

 course be doubtful. 



"We have now accounted for all the cells of these quadrants except 

 the four ventral cells of each quadrant, a^-^-c'-^, d}-"^ -c'-'^^a'-^-c'-^, and 

 a7.4_g7.4_ These correspond in origin to the entoderm cells of quadrant 

 D, and they do not cleave further until they are partly or entirely 

 enclosed within the embryo, as will be shown later. 



A diagram of that part of any one of the quadrants A, B, and C 

 corresponding to the ectodermal part of quadrant D, and showing the 

 conditions at the end of the seventh cleavage, is given on page 48 (Dia- 

 gram v.). A comparison of this diagram with that for the correspond- 

 ing stage of the quadrant D (Diagram lY. on the same page) shows 

 that the direction of the cleavage planes, and hence of the spindles, is the 

 same throughout in all cells of corresponding position, though there are 

 many differences as to the equality or inequality of the cleavage products. 



The general facts which may be deduced from the foregoing study of 

 the seventh cleavage in the ectodermal parts of the egg are similar to 

 those drawn from a study of the sixth cleavage, page 41. 



(1) Every cell of any quadrant divides with its spindle in the same 

 direction as the corresponding cell of any other quadrant. 



(2) All the cells in any layer cleave with spindles in the same direc- 

 tion (in spite of great differences in the form of the cells.) 



(.3) Xo general law can be deduced as to the equality or inequality of 

 the divisions. 



(4) There is a tendency for the largest cells to cleave fastest. Certain 

 very small cells (at the dorsal pole) do not cleave at all. 



Other Changes during the Seventh Cleavage. — In almost every cleav- 

 age which has taken place, whenever the division was equatorial, — the 

 spindles taking a dorso-ventral position, — it will have been noticed that 

 the axis in which the spindle was formed was the short axis of the cell. 

 On the other hand, the cells in which meridional cleavages have taken 



