234 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Planche VII., remembering that dorsal and ventral are reversed in Van 

 Beneden et Julin's figures.) The yolk-free protoplasm trails downward 

 from the attraction spheres forming a sort of crescent in each blasto- 

 mere. (See Plate III. Fig. 16.) 



During the second and third cleavages the nuclei remain somewhat 

 nearer the dorsal (vegetative) pole. (Plate IV. Figs. 20-22, and Plate V. 

 Fig. 30 ; cf. Van Beneden et Julin's ['84] Figs. 4 b and 5, Planche VII.) 

 It follows naturally that when the 8-eell stage is formed by the first 

 equatorial plane of cleavage (third cleavage), an inequality is observed 

 in the size of the newly formed blastomeres, the four nearer to the dorsal 

 pole being smaller than their sister cells, though the latter are richer in 

 protoplasm. (See Plate IV. Fig. 23, and Plate III. Fig. 18.) 



(b) Summary on Early Cleavage Stages. 



1. The future posterior end of the embryo is marked at the 2-cell 

 stage by an accumulation of protoplasm free from yolk in each blas- 

 tomere at contiguous regions. This accumulation persists throughout 

 cleavage, and forms at each cell division a pair of protuberances beyond 

 the general contour of the blastomeres. 



Subsequently to the 8-cell stage, in each of the two blastomeres in 

 which these accumulations lie, the spindle at three successive cell 

 divisions is directed toward the protoplasmic accumulation of that cell 

 and lies nearer to it than to the opposite side of the cell. In conse- 

 quence the newly formed cell, which contains the region in question, is 

 in each case smaller than its sister cell. (Cf. D 51 and D 52 , Fig. 38, 

 Plate VI. ; Z> 6 - 2 and Z> 6 - 3 , Fig. 51, Plate IX. ; and D 75 and Z> 7 - 6 , Fig. 62, 

 Plate X.) 



At each of these divisions also cleavage occurs later in the cells con- 

 taining the protoplasmic accumulations than in their sister cells. 



2. The first cleavage plane is vertical, and passes through the point of 

 formation of the polar globules. It coincides with the future median 

 plane of the embryo, and divides the egg into two blastomeres equal in 

 size and similar in every particular. They form respectively the right 

 and left halves of the embryo. The fate, as just stated, of the first two 

 blastomeres of the ascidian egg was first pointed out in the case of 

 Clavelina by Van Beneden et Julin ('84). 



3. The second cleavage plane is also vertical, and at right angles to 

 the first. Like the first, it passes through the point of formation of the 

 polar globules. It divides the egg into four blastomeres, among which 

 no difference of size can be recognized. 



