ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 19 



Corresponding with these positions of the spindles the subsequent divisions of 

 the cells are such as to lead to an inverse position of the cleavage cells in the 

 dorsal as compared with the ventral hemisphere. In the anterior-dorsal cells the 

 fourth cleavage furrows run from the anterior border of the cells to the transverse 

 (second cleavage) plane, and are approximately antero-posterior in direction: in 

 the posterior-dorsal cells these cleavage furrows run from the lateral borders of the 

 cells to the median (first cleavage) plane and are approximately transverse in direc- 

 tion. In the ventral hemisphere the reverse is the ease; in the anterior-ventral 

 cells the fourth cleavage furrows are approximately transverse in direction, in the 

 posterior-ventral cells approximately antero-posterior. Thus it comes about that 

 two of the anterior-dorsal cells do not reach the mid-line, while all of the posterior- 

 dorsal cells do; and that two of the posterior-ventral cells do not reach the mid- 

 line, while all of the anterior-ventral ones do. 



The fact that each hemisphere is thus the mirrored image of the other with 

 respect not only to the width of the anterior and posterior parts, but also as to 

 the direction of the fourth cleavage spindles and in the positions of the resulting 

 cleavage cells, this fact has contributed to the difficulties which most students of 

 ascidian embryology have experienced in distinguishing the dorsal and ventral 

 hemispheres, and has probably been responsible in some cases for the confusion of 

 those hemispheres. However, at this stage as at ever}- other, the two hemispheres 

 are easily distinguished by the relative amounts of yolk and protoplasm at the two 

 poles as well as by the position of the crescent and of the polar bodies. 



All of the cell divisions of this cleavage are approximately equal, except that 

 of the posterior-dorsal cells, B 41 and B 41 . These cells divide very unequally, giving 

 rise to two small posterior cells, B 52 and B 52 , which are the smallest in the entire 

 egg (figs. Ill, 113, 180). Since the work of Van Beneden and Julin. these cells 

 have been observed by all who have studied the ascidian cleavage, and they have 

 served as the most important landmark in the orientation of the cleavage stages. 



In this stage as in the preceding one. the yolk is most abundant in the cells 

 of the dorsal hemisphere; the protoplasm, in those of the ventral hemisphere; while 

 the yellow protoplasm is almost entirely confined to the posterior cells of the dorsal 

 hemisphere (figs. 36, 37, 38). In stained preparations the limits of the yolk and 

 protoplasm are sharp and distinct, and are represented in the drawings by a cre- 

 nated line (figs. 108, et seq.). The relative amounts of yolk and protoplasm at the 

 two poles can he readily seen by comparing figure 110 with 111, and 112 with 113. 

 The yolk and protoplasm of the four ventral cells are about equally distributed to 

 their eight daughter cells; the same is true of the two anterior-dorsal cells, which 

 divide so that each of their four daughter cells contains about the same proportion 

 of yolk and protoplasm (A 51 , A 5 - 2 , tigs. 36, 37, 113). However, in the division of 

 the posterior-dorsal cells, the daughter cells are qualitatively very disimilar ; the 

 small posterior cells (B 5 - 2 ) consist almost entirely of yellow protoplasm, while their 

 larger sister cells (B 5 - 1 ) are about half and half, yellow protoplasm and yolk. The 

 outlines of the yellow protoplasm or crescent are perfectly distinct as shown in 



7 JOUEN. -\. X. s. PHILA., VOL. XIII. 



