ORGANIZATION AM) CELL-LINEAGE OF ASCIDIAN EGG. 31 



than the others, must continue to divide more rapidly and thus give rise to the more 

 numerous ectoderm cells of the gastrular stage. So far from there being any dem- 

 onstration of this proposition there is actually no evidence offered in support of it. 

 Furthermore, I can affirm from my own studies that it is not true. The cells which 

 lag behind in division up to the 64-cell stage, thereafter divide much more rapidly 

 than the others and give rise to the ectoderm of the gastrula [cf. figs. 130134 and 

 196-204). 



(2) ( lastle's second reason for rejecting the orientation of Van Beneden and Julin 

 is the same as Samassa's, viz., the peculiar shape of the cells at the two poles. In the 

 32-cell stage and even earlier the cells at the maturation pole are long and columnar 

 while those at the opposite pole are thin and superficially lame. "They [the colum- 

 nar cells] retain this columnar form up to and throughout gastrulation " (1896, p. 

 237). They thus give rise directly to the columnar endoderm cells which are ulti- 

 mately invaginated. On the other hand. Van Beneden and Julin maintained that the 

 flattened cells of the 32-cell stage became the columnar cells of the 1 44-cell stage and 

 that the columnar cells of the earlier stage became the flattened ones of the latter 

 stage. Castle says that their figures show at a glance the absurdity of such an 

 interpretation (1894, p. 208; 1896, p. 237). Since the whole orientation which he 

 adopts as opposed to that of Van Beneden and Julin rests upon the establishment 

 of this one point, it passes belief that hi', as well as Samassa, should not have taken 

 the most evident and direct step to prove it. Van Beneden and Julin figure optical 

 sections in the sagittal plane of an egg in the 32-cell stage showing the columnar 

 cells at the ventral pole, and of one in the 44-cell stage showing them at the dorsal 

 pole. Castle figures actual sections of a 32-cell stage and of a 76-cell stage, but 

 none between these two. A study of actual or of optical sections of eggs transi- 

 tional between the 32-cell and the 76-cell stages would have shown conclusively 

 that the columnar cells of the former are graduall} transformed into the flattened 

 (ells of the hitter, and the flattened cells of the one into the columnar cells of the 

 other, and would thus have completely established Van Beneden and Julius 

 orientation. Such a series of optical sections of the Ciona egg, viewed from the left 

 side and also from the posterior pole, is shown in text figures IX to XVI, and the 

 various stages in this change of shape can there he clearly followed. A similar 

 series of actual sections of the egg of Cynthia is shown in text figures XVII to XXIV. 

 I do not find that this transformation is quite as rapid in Cynthia and Ciona as is 

 indicated by Van Beneden and Julin's figures 9 c and 10 c for Clavellina. At the 

 44-cell stage the cells at both poles are columnar and of nearly equal height (text 

 figs. XIII, XIV), and not until the 64-cell or even the 76-cell stage is this trans- 

 formation complete. It must not he supposed, however, that this change in shape 

 of the cells at the two poles is a continually progressive one. since all the cells he- 

 come more superficial during division and more columnar during rest. Consequently 

 every cell changes shape more or less during each cycle of division; this is well 

 shown in figures XVIII and XX. 



Other details which Castle regards as confirmatory of his view will lie taken 



