114 BULLETIN OF THE 



more elongated and conical in Ophiothrix than in Ophiopholis, and the 

 thin superficial layer of cells bearing the cilia is not represented in 

 Apostolides' figures.' The cavity of both is hollow. In Apostol ides' ♦ 

 figure of Ophiothrix we have in the middle, cell-like structures lettered, 

 es. He does not explain the lettering, but from the fact that he speaks 

 of the cavity as " creux," it is supposed that this region is a cavity, the 

 segmentation cavity. In a copy of this figure in Embryological Mono- 

 graphs t A. Agassiz letters the cells of the blastoderm ; e, ectoderm, and y 

 '• yolk cells." The structures y are the same as es. 



In a comparison of our figures of a blastosphere, PL I. figs. 10, 11; 

 with that of Apostolides, we see in both a slight infolding of the blasto- 

 derm, which is here regarded as the beginning of the invagination in 

 both cases. Apostolides does not so consider it in Ophiothrix, but he 

 ascribes to Balfour the mistake of considering it an infolding. He says : 

 " C'est peut-etre ce point que M. Balfour, qui n'a pas pousst^ tres loin ses 

 observations, a pris pour un commencement (^invagination. II n'en est 

 pourtant rien, la suite prouvera que ce point n'est que le premier indice 

 de la formation des bras du pluteus." It is a significant fact that just 

 between this stage (his fig. 9) and the stage which he figures in fig. 10, 

 when cal6areous rods are developed, is the time when the process of 

 invagination occurs. I find no stages of Ophiopholis which resemble in 

 shape his figures 10 and 11 of Ophiothrix. 



Apostolides says : " Peut-etre M. Balfour a-t-il obtenu des f^conda- 

 tions de VOphiotrix rosula, qui est plus abondante en Angleterre, et 

 chez laqueUe les choses se passent peut-etre autrement que dans I'espfece 

 que nous avons souraise k I'observation." It would be an interesting 

 fact if one species of Ophiothrix forms a gastrula stomach by infolding, 

 and another in the way described by Apostolides. J Closely related star- 

 fishes, sometimes regarded as simply different species, however, have a 

 wide difference in their development. A. vulgaris has a brachiolaria, 

 while Leptasterias has young without nomadic stages. The gastrula of 

 the latter may or may not develop as that of the former. There is noth- 

 ing to show that it is exceptional. 



The " plan general " of the development of the gastrula of Echinoderms 

 is more widely soread among Echinoderms than the following quotation 



* Op. cit, PI. XI. fig. 9. 



t Mem. Mua. Comp. ZoUl, Vol. IX. No. 2. 



X We are here brought face to face with one serious defect in Apostolides' and 

 Balfour's observations, namely, the difllculty of knowing exactly the species which 

 both studied. 



