32 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
up later, but enough has now been said, in my opinion, to.show the untrustworthi- 
ness of his principal evidence against Van Beneden and Julin’s system of orientation 
and in favor of his own. 
9. Evidences in favor of Van Beneden and Julim's System. 
While it is evident from these many and serious differences of opinion that it 
is easy to make mistakes in the orientation of the ascidian egg, it is not true that 
the egg is ап unusually difficult one to orient. In fact there are few eggs, except 
веј B 
those in whieh the cleavage is markedly unequal, in which this can be so easily 
done. All the embryonic axes are clearly distinguishable in the unsegmented egg, 
and at every stage in development there are numerous landmarks by which the 
different poles of the egg may be recognized. With the exception of Seeliger, all 
students of the early development of ascidians have recognized that from the 16- 
cell stage onward, the posterior pole is marked by two cells much smaller than 
.any others in the entire egg. Тһе chief difficulty has been, as evidenced by the 
