ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



25 



plasmic fields quite separate. The two spindles are usually parallel to each other 

 and are always entirely independent, the pules never being united into a triaster 

 or tetraster. 



Sections of two dispermic eggs are shown in figures 94 and 95; in the former 

 the sperm nuclei, which have not yel reached the equator of the egg, occupy symmet- 

 rical positions on each side of the mid-line, and the protoplasmic field in which the} 

 lie is located on that side of the egg which corresponds to the posterior pole of nor- 



FlGS. III-VI. Dispermic eggs of ('. partita; drawn from stained preparations of entire 

 eggs. Figs. III. and V are viewed from the vegetal pole, the polar bodies being seen 

 through the egg. Fig. IV is viewed from the animal pole and Fig. VI from the posterior 

 pole. The boundary between the protoplasm and yolk is indicated by a crenated line; 

 when seen through the egg this boundary is represented by a line of stipples. 



mal eggs. The yellow protoplasm here forms a continuous crescent, and save for the 

 fact that the sperm nuclei do not lie at the middle of this crescent and that a small 

 tongue of yolk partly separates the two sperm asters, the egg is not unlike a normal 

 one. In figure 95 a later stage of a dispermic egg is shown, in which the sperm nuclei 

 have reached the equator and have moved in from the surface toward the center of 

 the egg, while one of these nuclei has united with the single egg nucleus. There 

 is here also a symmetrical arrangemenl of the sperm nuclei and of the (dear and yel- 

 low protoplasm on each side of the mid-line. The protoplasmic areas are here further 



4 JOURX. A. X. S. PHILA., VOL. XIII. 



