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 C^-\ D'-^ (Plate XL 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 pi'ominences 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 : " II est encore une marque propre aux cellules 

 P et P [^*'S C*--^] que permet de les distinguer de toutes les autres, elle 

 consiste en une petite saillie en forme de mamelon, saillie qui est dirigee 

 horizontalement en arriere et que montrent les figures 2 et 23 de la 

 planche XVIII. Cette saillie n'est visible qu'au debut du stade VIII. 

 [S-cell] et surtout durant la segmentation qui produit P et P [Z>^■^ C'*-^]." 

 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 x*efei-able to a supposed 

 general phenomenon, which, stated in his own words, is as follows: " Que 

 les blastomeres out a I'instant oii ils viennent de se produire et mieux 

 encore durant leiir individuallsation des formes specifiques qu'ils perdent 

 peu d'instants apres. Ces formes specifiques paraissent etre en rapport 

 avec les segmentations dont ces blastomeres seront plus tard le siege . . . 

 la segmentation a done lieu dans tons les cas, perpend iculairement au 

 plus grand axe que possedaitle blastomere durant son individuallsation." 



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 the time of their formation ("individuallsa- 

 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.^ 



1 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 tilings being er/nul, it is true that the spindle 

 arises in the longest axis of the cell. 



2 I am 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 



