ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 37 



identify them they have been found at the same polo of the egg as in Ciona (figs. 

 87. 92, 96 3 102, 106, HIT. ins. 1 It). I ir,. I hi. L30, L33, L39, I 13). In stages later 



than figures L39 and I !"> 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 1 have already shown, that the 

 polar bodies of Ciona are formed at the middle of the endodermal or dorsal half of 

 the egg. I have never in a simile instance observed anything which might he 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 genera] 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 Castle's figures correctly drawn, and I can without diffi- 

 culty correlate his drawings cell for cell with my own. The most important ex- 

 ceptions to this statement are found in his figures 53 and 54, hut 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. All of his pregastrular stages, however, with the exception 

 of a 48-cell and a 64-cell stage, shown in his figures 57 to GO. are erroneously 

 oriented, dorsal being mistaken for ventral and ventral for dorsal. With the 

 lineage which he gives of every cell up to the 4G-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 r 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. 



*o^> 



IV. CELL-LINEAGE. 

 A. Nomenclature. 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 he not ideally perfect. The 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 



