56 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



Turning now to a detailed study of the observations of Samassa and Castle 

 during this critical sixth cleavage, we find that Samassa did not attempt to follow 

 the cell-lineage further than the 48-cell stage (his tig. 9), but jumped at once from 

 this stage to one with at least 76 cells (his fig. 10). His orientation of all stages up 

 to and including the 48-cell stage (his fig. 9) is the reverse of that of Van Beneden 

 and Julin, and is wrong; his orientation of the gastrula, shown in Lis figure 10, is 

 right. Therefore, in the interval between his figures 9 and 10 he has inverted the 

 egg so that the dorsal face of his figure 10 corresponds to the so-called ventral face 

 of all preceding figures. 1 



Castle, on the other hand, has traced the cell-lineage much further than the 

 48-cell stage, and it is therefore possible to follow in detail the manner in which lie 

 passes from the erroneous orientation of earlier stages to the correct orientation of 

 later ones. He has given correctly the lineage of every cell up to and including 

 the 46-cell stage (his figs. 55 and 5(i), as I have convinced myself by comparing his 

 figures, cell tor cell, with my own. but his orientation of these stages should be 

 reversed. On the other hand his orientation of all stages later than the 46-cell 

 stage is correct, but the cell lineage of these stages is wrong. This is due to the 

 fact that between the 46-cell and the 48-cell stages (his figs. 56 and 57) he has 

 inverted the egg so that the dorsal surface of all stages later than the 46-cell stage 

 corresponds with the so-called ventral surface of all earlier stages. 2 This inversion 

 of the egg introduces many profound errors in the cell-lineage after the 46-cell stage. 



Considering in detail Castle's account of this sixth cleavage we find that be 

 has correctly represented the divisions of the cells of the real dorsal hemisphere 

 which bring these cells up to the seventh generation and the entire egg up to the 

 46-cell stage (his fig. 55). At this stage the cells of the ventral hemisphere are 

 still in the sixth generation (:-. his fig. 56). and this stage is almost exactly compara- 

 ble with my figures 1 1 9 to 1 23. Immediately after this, in the 4S-cell stage (his figs. 



57 and 58), Castle supposes that the cells of the real dorsal hemisphere, which are 

 now in the seventh generation, divide again, thus passing into the eighth generation, 

 while the sixth generation cells at the opposite pole are supposed by him to remain 

 undivided. It is absolutely essential to his scheme of orientation that the cells of 

 one hemisphere should remain in the sixth generation, while those of the other hemi- 

 sphere are advancing to the seventh and eighth generation. If it could be shown 

 that all the cells of both hemispheres divide during this sixth cleavage it would com- 

 pletely break down Castle's orientation of the earlier stages and his cell-lineage of the 

 later ones. In all of his figures of this cleavage (figs. 55, 56. 57, 58) Castle represents 

 the cells at one pole in process of division while those at the other pole are in the 

 resting condition. However, in two of my figures of this cleavage in Ciona (figs. 



1 In his explanation of figures he says that figure '> is viewed from the cephalic pole; this is, .if 

 course, a verbal error, since his lettering of the cells shows plainly that the egg is viewed from the 

 caudal pole. 



2 Unfortunately Castle gives no dorsal, ventral nor lateral views of this critical 48-cell stage at 

 which the inversion occurs, but only an anterior and a posterior view (his tigs. 57 and 58, reproduced in 

 text tigs. XXV and XX VI (if this paper). 



