ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 57 
identify them they have been found at the same pole of the egg as in Crona (figs. 
87, 92, 96, 102, 106, 107, 108, 110, 115, 116, 130, 133, 139, 143). In stages later 
than figures 139 and 143 the protoplasm of the polar cells becomes vesicular and 
stains so faintly that they can no longer be identified with certainty. 
Castle maintained from indirect evidence, as I have already shown, that the 
polar bodies of Crona are formed at the middle of the endodermal or dorsal half of 
the egg. I have never in a single instance observed anything which might be mis- 
taken for a polar body at this pole, whereas I have found the most positive and oft 
repeated evidence that the polar bodies lie at the ectodermal or ventral pole from 
the time of their formation to the gastrular stage. "These ascidians therefore form 
no exception to the general rule that the polar bodies are formed at the middle of 
the ectodermal hemisphere of the egg. 
It is not necessary in this place to point out in more detail than has been given 
already the sources of error in the work of Seeliger and of Samassa, especially as their 
work does not undertake to follow the exact cell-lineage of every cell up to the 
gastrular stage or later. With Castle's work, however, the case is quite different, 
for while the considerations already mentioned probably explain the sources of his 
error of orientation, they do not explain the way in which he has incorporated this 
error in the cell-lineage which he has followed to an advanced stage. In brief, 
I find almost all of Castles figures correctly drawn, and I can without diffi- 
culty correlate his drawings cell for cell with my own. Тһе most important ex- 
ceptions to this statement are found in his figures 53 and 54, but even here the dif- 
ferences are not great. His gastrular stages are of course correctly oriented since the 
dorsal and ventral faces of the embryo are unmistakably marked out as soon as the 
invagination begins. АП of his pregastrular stages, however, with the exception 
of à 48-cell and a 64-cell stage, shown in his figures 57 to 60, are erroneously 
oriented, dorsal being mistaken for ventral and ventral for dorsal. With the 
lineage which he gives of every cell up to the 46-cell stage (his fig. 56), I entirely 
agree, but in passing to the 48-cell stage (his figs. 57 and 58) he inverts the egg 
and shifts the equator one cell-row nearer the vegetal pole than it should be, 
consequently all of the lineage of the later stages is wrong. While therefore 
the stages from 48 cells on are correctly orientated, the lineage of the individual 
cells is incorrect; before the 48-cell stage the lineage is correct but the orientation 
wrong. The evidence for this grows in part out of the general considerations 
already mentioned, but it is also founded upon a detailed study of the cell-line- 
age, to which we now turn. 
IV. CELL-LINEAGE. 
A. NOoMENCLATURE.—[In order to facilitate reference to the work of others, it 
is desirable that some good system of naming the individual cleavage cells be 
adopted and thereafter adhered to even if it be not ideally perfect. Тһе system 
which has been employed with only slight modifications in all the recent cell-line- 
age work on annelids and mollusks is not well suited to the ascidian egg because in 
