16 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



During telokinesis the movements in the cell body are similar to those which 

 occur at the close of the second cleavage, i. e.. the middle of the spindle is carried 

 in toward the centre of the egg while the poles of the spindle move outward toward 

 the surface (figs. 106, 107). By this movement the spindle axis is much bent on 

 itself. I have not observed in these eggs any tendency for the sphere substance at 

 the poles of the spindles to be carried as near as possible to the animal pole, a 

 thing which is very apparent in gasteropod eggs. 



I have already called attention to the fact that the four cells at the animal pole 

 are smaller than those at the vegetal pole; this disparity is most marked between 

 the upper and lower cells of the anterior quadrants (figs. 106, 107). The anterior 

 dorsal cells (A 41 ) are the largest in the egg at the eight-cell stage, the anterior 

 ventral cells (a 42 ) the smallest. The posterior dorsal cells (B 41 ) are but little, if any, 

 larger than the posterior ventral ones (h 42 ), and both are intermediate in size 

 between the upper and lower anterior cells. 



The different cell substances are distributed to the eight cells as follows : The 

 clear protoplasm is found in all the cells, but is most abundant in the four ventral 

 cells and least abundant in the two posterior dorsal cells (B 41 ); yolk is found in 

 all of the cells, but is most abundant in the two anterior dorsal cells (A 41 ) and least 

 abundant in the four ventral cells ; the yellow protoplasm or crescent substance is 

 confined almost entirely to the two posterior dorsal cells (B 41 ), but a very small 

 amount of it is found around the nuclei of all the cells (figs. 106, 107, Cr. s.). My 

 attention was first drawn to the yellow protoplasm around the nuclei in my study 

 of the living eggs of Cynthia (figs. 32-40, et seq.)\ since then I have found, in 

 preserved material, a few of the spherules of this yellow protoplasm around the first 

 cleavage spindle and around the resting nuclei of the 2-cell, 4-cell and 8-cell stages 

 (figs. 96, 106, 107). In later stages of development it is found around the nuclei 

 of a few of the ectoderm cells even as late as the young tadpole stage (plates IV 

 and V). In spite of this perinuclear distribution of some of this crescent substance, it 

 is largely limited to the two posterior dorsal cells (B 41 ) of the 8-cell stage, where it 

 constitutes more than half of all the substance of those cells (figs. 31-35, 106, 107). 



Van Beneden and Julin lirst observed that the four ventral cells of the 

 8-cell stage are smaller than the lour dorsal ones; Seeliger, Samassa, and Castle 

 observed this same fact, though they incorrectly called these smaller cells dorsal in 

 position. Castle is quite right when he says (1896, p. 228) the " four cells which Lie 

 nearest the polar globules are smaller than those more remote," but I cannot under- 

 stand how it was possible for him to reach the conclusion that- the smaller cells 

 ' are more abundantly supplied with yolk," while the larger cells are richer in pro- 

 toplasm (pp. 234 and 235). According to my observations this is not true of Cz'ona, 

 Cynthia or Molgnla. 



During the 8-cell stage two little bosses or caps of clear but deeply staining 

 protoplasm which will give rise at the 64-cell stage to the small posterior mesen- 

 chyme cells (B 7,; fig. 130 et seq.) are visible on the posterior surface of the cells B 41 

 and B 4 ' (fig. 184). These little caps lie in contact with each other on each side of 



