CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 231 
hemisphere. The persistence of this clear polar region in stages later 
than that of 24 cells was shown in certain figures of my preliminary 
paper (reproduced in Plate IX. Figs. 54 and 55). It finally passes into 
the small flattened cells 075, D'5 (Plate XI. Fig. 71), of whose later 
history we shall have more to say. 
Chabry (87) observed in Ascidiella at the beginning of the 8-cell 
Stage the formation of polar prominences such as I have described, 
and spoke of them as a sure means of orienting the egg at this stage. 
On page 203 he says: “Il est encore une marque propre aux cellules 
P et P | D, C*"] que permet de les distinguer de toutes les autres, elle 
consiste en une petite saillie en forme de mamelon, saillie qui est dirigée 
horizontalement en arriére et que montrent les figures 2 et 23 de la 
planche XVIII. Cette saillie n'est visible qu'au début du stade VIII. 
[8-cel1] et surtout durant la segmentation qui produit P et P I 
Apparently Chabry overlooked the formation of the prominences at 
other than the 4- and 8-cell stages, and failed to recognize their true 
significance. For he explains them as merely foreshadowing the form 
and direction of the next cell division, and as referable to a supposed 
general phenomenon, which, stated in his own words, is as follows: * Que 
les blastoméres ont à l'instant od ils viennent de se produire et mieux 
encore durant leur individualisation des formes spécifiques qu'ils perdent 
peu d'instants aprés. Ces formes spécifiques paraissent être en rapport 
aveo les segmentations dont ces blastoméres seront plus tard le siège . . 
la segmentation a done lieu dans tous les cas, perpendiculairement au 
plus grand axe que possédait le blastomere durant son individualisation." 
It is hardly necessary, I suppose, to say anything at this late day in 
refutation of Chabry's generalization. My own observations indicate 
that cells tend to assume at tho time of their formation (* individualisa- 
tion”) a spherical form," if they are homogeneous in structure, and that 
the departure from an evenly rounded contour at the posterior end of the 
ventral hemisphere is explicable by the presence there of a region pecu- 
liar in its constitution, containing as it does less yolk than the other 
superficial portions of the egg.? 
' Mutual pressure of cells may modify this form, in which case the direction of 
the next division may perhaps be predicted, as Chabry states, at the time of the 
" individualisation ” of cells. For, other things being equal, it is true that the spindle 
arises in the longest axis of the cell. 
2 Lam aware that Van Beneden et, Julin (84) have offered an entirely differ- 
ent explanation for certain phenomena probably related to those under discussion, 
which they observed in the cleaving egg of Clavelina, Their explanation implies 
