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 fig. 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 his 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.' | 
Castle, on the other hand, has traced the cell-lmeage much further than the 
48-cell stage, and it is therefore possible to follow in detail the manner in which he 
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 56), as I have convinced myself by comparing his 
figures, cell for 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? 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 he 
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 ege up to the 
46-cell stage (his fig. 55). At this stage the cells of the ventral hemisphere are 
still in the sixth generation (v. his fig. 56), and this stage is almost exactly compara- 
ble with my figures 119 to 123. Immediately after this, in the 48-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 Crona (figs. 
1 In his explanation of figures he says that figure 9 is viewed from the cephalic pole: this is, of 
course, a verbal error, since his lettering of the cells shows plainly that the egg is viewed from the 
caudal pole. a 
* 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 figs. 57 and 58, reproduced in 
text figs. XXV and XXVI of this paper). 
