ORGAN-FORMING SUBSTANCES IN EGGS OF ASCIDIANS. 213 



cells this perinuclear plasm retains these positions and therefore 

 goes into certain daughter cells and not into others. Consequently 

 even in those cells in which it is found very sparingly this yellow 

 protoplasm is localized with great definiteness. 



In the i6-cell stage all these different substances remain in 

 exactly the same positions which they held at the 2-, the 4- 

 and the 8-cell stages (Photos 12-14). The eight ventral cells 

 are all similar in appearance and are composed chiefly of clear 

 protoplasm. The yellow crescent still surrounds the posterior 

 half of the dorsal hemisphere, as in the i-cell stage, but it is now 

 contained in four cells ; it occupies the posterior and lateral por- 

 tions of these cells while the anterior and median portions are 

 composed of gray substance. The boundary between these two 

 substances could not be sharper if they were actually, as at first 

 sight they seem to be, separated by a cell wall (Photo 14) ; only 

 at the next stage, however, are these substances segregated into 

 separate cells. The gray crescent occupies the anterior portions 

 of the four anterior cells of the dorsal hemisphere, the posterior 

 portions of these cells being composed of deep gray (endodermal) 

 substance (Photo 14). 



At the 32-cell stage (Photos 15, 16) the substances of the yel- 

 low and the gray crescents are finally segregated into separate 

 cells, although a small portion of deep gray substance is still 

 contained in the median cells of the yellow crescent ; however 

 this grav material moves in from the surface so that it does not 



o * 



show in photographs of the entire egg. The yellow crescent 

 now consists of six cells, four median ones which are small and 

 one pair .of lateral ones which are relatively large (Photos 15, 16). 

 The gray crescent consists of four cells of equal size and similar 

 constitution. The ventral half of each cell is clear, contains 

 little yolk and gives rise to most of the neural plate, the 

 dorsal half is light gray in color, is yolk-laden and gives rise to 

 the chorda. From this stage onward the ventral hemisphere is 

 composed of clear cells, all of which are much alike ; for this 

 reason no photographs are given of the later stages of these cells. 

 In subsequent stages all of the cells of the gray and yellow 

 crescents divide in a vertical direction (parallel to the egg axis) ; 

 these divisions occur first in the gray crescent and in the most 



