58 ORGANIZATION AND CELL-LINEAGE OF 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 

 hand 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 

 lie shown b^' a detailed stud \ 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- 



CastU 



Conklin 



Castle 



o 



o, 



lA" 



A". 



o 



a 6 s 



XXV 



XXVI 



Figs. XXV and XXVI. Surface views of eggs of Cionn intesHnalis; copied from Castle's figures 5T and 

 58 (1896). Fig. XXV represents an anterior view ; Fig. XXVI a posterior one of the same egg. Tlie orienta- 

 tion and cell-lineage, according to Castle, are indicated by the designations of the cells in that half of the egg 

 under his name; the designations of the corresponding cells in the other half of each figure shows the system 

 of orientation and cell-lineage adopted in this paper. Owing merely to differences in nomenclature the cells 

 in the right half of Fig. XXVI are designated by the letter D, those on the left by the letter B. Every- 

 where lower-ease 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 oi 



HV C 



lis which retain a semblance of their former names 



throughout this revolution arc the small posterior cells (C 75 . D 75 of Castle's system). 

 and their sisters (C 7,li . D"' 1 ), the most anterior cells of the crescent of each side (C 74 , 

 D 7 - 4 ), and the most anterior pair of cells of the dorsal hemisphere (A 7-4 , B 74 ). 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 designations for 

 given cells. For example, the cell which in his figure 58 is labelled A 7-8 becomes 

 a 6,7 in fig. 00 ; a 6-7 of figure 58 becomes d 6 - 5 of figure GO, while the one labelled d' ! - 5 

 in the former figure becomes A 7-6 in the latter. Strangely enough this last cell 

 which had been variously located in the dorsal and ventral hemispheres, and in the 



