40 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



exactly in the center of the egg (fig. 96), always heing oriented so that it lies at 

 right angles to the plane of the first cleavage and with its equator in that plane. 

 At the same time the clear protoplasm and a small portion of the yellow protoplasm 

 move inward from the posterior pole toward the center of the egg (figs. 92, 96). 

 . The larger part of the yellow protoplasm remains at the surface in the form of a 

 crescent, but the clear protoplasm is entirely withdrawn from the surface except for 

 a narrow zone which lies just above (ventral to) the crescent on the posterior side 

 (figs. 13-18, 96). During the formation of the first cleavage furrow, even this 

 narrow zone of clear protoplasm is withdrawn from the surface to the center of the 

 egg, so that the yolk now covers the entire surface of the egg except for the area 

 of the crescent (tigs. 100-102 and 178-179). This condition is just the reverse of 

 that which prevailed at the beginning of development, when the yolk was central 

 in position and the protoplasm peripheral (figs. 76-79). 



The centrosomes and asters are larger and more easily studied in Ciona than in 

 Cynthia. Proceeding from the periphery to the center, the following parts of the aster 

 may be recognized (figs. 177, 179) : (1) The deeply staining, peripheral layer of the 

 aster, (2) the clear inner layer of the aster traversed by radiating fibres, (3) a granular 

 central body upon which the astral fibres end. The latter is the centrosome, and 

 is plainly composed of two parts, (a) an outer granular zone and (b) a central clear 

 area from which the netrum arises. In Cynthia the outer and inner layers of the 

 aster are not distinguishable and the centrosome itself is not so large as in Ciona ; 

 the latter is, however, composed of the same parts, viz., a peripheral granular zone 

 and a central clear area which gives rise to the netrum (figs. 98, 99). In these 

 ascidians, just as in the gasteropods which I have studied (Conklin, 1902), the 

 centrosome undergoes a decided growth and metamorphosis during the cycle of 

 division ; in the early stages of the cycle it is a small, deeply staining body, in the 

 Later stages it becomes much larger and differentiates into the outer granular zone 

 and the central clear area (cf. figs. !)7, 98). 



In Ciona both the first and second cleavage spindles are remarkable in that at 

 all stages of the division the nuclear part of the spindle can be clearly distinguished 

 from the polar or astral part (figs. 177, 17'.l). The portion of the spindle derived 

 from the linin of the germ nuclei is short, deeply staining and barrel-shaped, and in 

 all respects resembles a maturation spindle (compare figs. 177 and 179 with figs. 67. 

 70, 71). Even in the possession of a few peripheral fibres which radiate from the 

 slightly rounded ends of the spindle toward the equator, this spindle resembles 

 those of the maturation divisions. These peripheral fibres are not in line with the 

 astral radiations, and hence are all the more striking. The astral rays which run 

 from the centrosomes to the ends of this nuclear spindle are small and faintly stain- 

 ing as contrasted with the heavy, deeply staining fibres of the nuclear spindle. No- 

 where else, so far as I am aware, is this double character of the mitotic figure so 

 clearly shown as in these cleavages of Ciona. This is due to the small size of the 

 nuclear spindle and to the large size of the astral systems, so that the ends of the 

 nuclear spindle are not easily confused with the astral rays, and also to the great 



