58 ORGANIZATION AND CELL-LINEAGE ОЕ ASCIDIAN EGG. 
As it can be proved that no cells of this stage remain in the sixth generation, 
but all have passed into the seventh, it is certain that the equator both here and in 
the 48-cell stage is in the wrong place, that it really lies between his equatorial 
band and the ectodermal group, and that there are therefore thirty-two cells in each 
hemisphere in the 64-cell stage. 
Wholly apart, therefore, from the perfectly conclusive evidence as to the orien- 
tation of the egg and embryo which may be drawn from the histological character 
of the cells at the two poles, as well as from the location of the polar bodies, it can 
be shown by a detailed study of the cell-lineage that Castle has inverted the egg at 
the 48-cell stage, transferred sixteen cells from the dorsal to the ventral hemisphere 
and consequently shifted the equator of the embryo at least one cell row nearer the 
dorsal pole than it should be. Of course the lineage of every cell is thereby pro- 
Castle Conklin Conklin Castle 
IGS. XXV and XXVI.—Surface views of eggs of Ciona intestinalis; copied from Castle’s figures 57 and 
58 (1896). Fig. XXV represents an anterior view; Fig. I a posterior one of the same egg. e orienta- 
tion and cell-lineage, according to Castle, are indicated by tbe designations of the cells in tbat half of the egg 
4 
Ф 
п 
іп the right half of Fig. XXVI are designated by the letter D, those on the left by the letter В 
where lower-case letters designate cells of the animal (maturation) hemisphere ; capitals, cells of the hemisphere 
opposite the maturation pole. The equator lies between the cells designated by lower-case and capital letters. 
foundly changed; the only cells which retain a semblance of their former names 
throughout this revolution are the small posterior cells (C“*, Т) of Castle's system), 
and their sisters (C^^, 0"), the most anterior cells of the crescent of each side (C74, 
D*^), and the most anterior pair of cells of the dorsal hemisphere (А74, B™4), Even 
in the case of these four pairs of cells the right and left cells of each pair are inter- 
changed, so that everywhere A should replace B, and C, D. 
In subsequent stages Castle does not always preserve the same—esignations for 
given cells. For example, the cell which in his figure 58 is labelled А"? becomes 
a^ in fig. 60; аё of figure 58 becomes 4% of figure 60, while the one labelled d^ 
in the former figure becomes А? in the latter. Strangely enough this last cell 
which had been variously located in the dorsal and ventral hemispheres, and in the 
