.>_' 



ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



up Inter, hut enough lias now been said, in my opinion, to show the unt rust worthi- 

 ness of his principal evidence againsl Van Beneden and Julin's system of orientation 

 and in favor of his own. 



5. Evidences in favor of Van Beneden and Julias 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 an unusually difficult one to orient. In fact there are few eggs, except 



Figs. IX-X1I. Camera drawings of entire stained eggs of Ciona iiitesthuiUs viewed as transparent 

 objects. Figs. IX and XI are seen from the posterior pole; Figs. X and XII from the left side. Figs. 

 IX and X represent a 16-cell stage passing into a 24-cell stage ; Figs. XI and XII a 32 -cell passing 

 into a 44-cell stage. The head and tail of the arrow mark the position of the equator (third cleavage 

 plane) at the anterior and posterior poles. The cells of the crescent (mesoderm) are B 6 -3, B 6 -> and B 6 = ; 

 all the other cells of the lower hemisphere are yolk laden and the boundary between protoplasm and 

 yolk is indicated by a crenated line; the stippled areas adjoining the median plane in the cells Bs- 2 and 

 B 6 -3 represent caps of deeply staining protoplasm (clear in life). The segmentation cavity is shaded by 

 vertical lines and the cells bordering it are seen in median optical section ; the cells at the upper pole 

 are columnar, those at the lower pole flattened. The polar bodies, although shaded diagramatically, are 

 present exactly where they appear in the drawings. 



those in which the cleavage is markedly unequal, in which this can be so easily 

 done. All the embryonic axes are (dearly 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. The chief difficultv has been, as evidenced by the 



