ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 13 



the attraction spheres and nuclei begin to move toward the dorsal surface of the 

 egg, me ay fro?) i its more richly protoplasmic animal pole, from which the plane of 

 separation cuts it more rapidly." I find, however, that in Ciona, as in Cynthia, 

 thi< telokinetic movement of the nuclei and spheres is not away from the more 

 richly protoplasmic pole, unless the substance of the crescent he considered as con- 

 stituting the protoplasmic pole, hut that the clear protoplasm also moves with the 

 nuclei and spheres toward tin' animal (ventral) pole (fins. ITS. 182). Sobotta (1807) 

 has also observed a similar bending of the spindle axis and movement of the 

 daughter nuclei and centrosomes in the anaphase or telophase of the first cleavage 

 of the egg of Amphioxus. He describes it as an attempt on the part of the centro- 

 somes and nuclei to regain the center of the blastomeres ; but it is probable that this 

 is only another case of the telokinetic rotation of the cell contents with the conse- 

 quent establishment of a new cell axis [cf. Conklin, 1902). 



2. Second Cleavage; 2-4 cells. (Figs. 28-30, 101-105. 179, 182, 183). 



During the anaphase of the first cleavage each centrosome becomes elongated 

 at right angles to the spindle axis and to the chief axis of the egg and gives rise to a 

 minute centrosomal spindle or netrum at each pole of the mitotic figure (figs. 98, 99). 

 These netra elongate in the antero-posterior axis until the daughter centrosomes 

 come to lie at the anterior and posterior poles of the nucleus of each blastomere ; the 

 nuclear membrane is then dissolved and the second cleavage spindles are formed (figs. 

 101, 102, 177, 179). These spindles, like those of the first cleavage, are composed 

 of a nuclear and an astral portion, the two being distinguishable with especial ease 

 in Ciona (fig. 179). The spindles lie in an area of clear protoplasm above the equa- 

 tor of the egg, and in Cynthia are slightly eccentric toward the posterior pole (fig. 102). 

 The areas of clear protoplasm become elongated in the antero-posterior axis (figs. 28, 

 101, 102, 179) ; they are surrounded on all sides by yolk, which forms a peripheral 

 laver over the whole surface of the egg, except in the region of the crescent. The 

 substance of the crescent is directly continuous with the clear protoplasm in the 

 region of the posterior pole of the second cleavage spindle (figs. 102, 17'.)). Although 

 the yolk surrounds the areas of clear protoplasm it is not uniformly thick on all 

 sides; on the upper or ventral side of these areas the layer of yolk is very thin, on 

 the lower or dorsal side and anterior to the crescent it is especially thick. This dis- 

 tribution of the yolk can be seen not only in sections, such as figures 102 and 17!), but 

 also in entire preparation and in living eggs. In the latter the light gray of the 

 upper hemisphere (figs. 28, 29 and 30), as contrasted with the dark gray of the lower 

 hemisphere, indicates that the layer of yolk which surrounds the areas of clear pro- 

 toplasm is thin over the upper hemisphere and thick over the lower. The area at 

 the vegetal pole, where the layer of yolk is thickest, gives rise to the endoderm cells 

 which are always yolk laden : the upper hemisphere, where the yolk layer is thin. 

 gives rise to the ectoderm cells which contain a relatively small amount of yolk ; 

 the substance of the crescent gives rise to the muscle and mesenchyme cells, and in 

 Cynthia is always characterized by the presence of the yellow spherules. In later 



