CASTLE. — CELL LINEAGE OF THE ASCIDIAN EGG. 207 



it visible. This fact does uot, however, foreshadow an immediate di- 

 vision of the cell, as these radiations often exist some time before the 

 division actually occurs. It does indicate, however, as the sequel will 

 show, that this cell with its mate, a^-^, will be among the earliest 

 cells ot the eiidodermal half of the egg to divide. 



In Figures 9 and 10 are reproduced Van Beneden et Julin's fig- 

 ures of the 44-cell stage of Clavelina (their Figs. 10 and 10a). The 

 lettering on the left half of Figure 9 and the right half of Figure 10 

 indicates what I believe to be the correct interpretation of the cell 

 lineage of this stage. The interpretation of Van Beneden et Julin 

 is indicated (in terms of my own nomenclature) on the right half of 

 Figure 9 and the left half of Figure 10.* 



It .will be seen that at this stage, according to my interpretation, 

 exactly the same divisions, with one exception, have occurred in the 

 egg of Clavelina as in the Ciona egg shown in Figures 5 and 6. 

 The exception mentioned is m the case of the most anterior cell of the 

 ectodermal half, on each side of the median plane (A'^'^ and its mate, 

 Figs, 8 and 9). This cell has not yet divided, but has remained 

 in the sixth generation, whereas in Ciona it has given rise to A^-^ 

 and A^ ■*. 



Not only has division occurred in corresponding cells in the egg of 

 Clavelina and that of Ciona, but this division has taken plate in the 

 same direction in every instance. 



There is, however, in the two eggs, a slight difference in the ar- 

 rangement of the cells resulting from this division, due perhaps to a 

 difference in the order of division. In the egg of Clavelina (Fig. 9, 

 left half), the cells A''^ and D'^"^ have been pushed toward the me- 

 dian plane so that they are in contact with their mates in the other 

 half of the egg, and separate A"-^ from D"'-^ ; but in the egg of Ciona 

 (Fig. 5) A'^*^ and D'''' do not reach the median plane, so that A^-^ is 

 left in contact with D'^*^. 



Van Beneden et Julin identify the face of the egg represented in 

 Figure 9 with that seen in Figure 8, and that in Figure 10 with the 

 one seen in Figure 7. In doing so they are forced into several very 

 strange correlations. For example, the small posterior cell C®'^ of 

 Figure 8 is identified with the rather large cell D'^-^ of Figure 9 ; 

 while the real C^-^ (D^-^ of Fig. 10, left half) is derived from D^-^ 

 (Fig. 7), and its sister cell is supposed to have divided again, so that 



* It will facilitate a comparison with the lettering of the original, if the 

 reader will make use of the table on page 204. 



