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 Apostolides’ * 
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. Ina copy of this figure in Embryological Mono- 
graphs f 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-étre ce point que M. Balfour, qui n’a pas poussé tres loin ses 
observations, a pris pour un commencement d’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 calcareous 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-étre M. Balfour a-t-il obtenu des féconda- 
tions de l’Ophictrix rosula, qui est plus abondante en Angleterre, et 
chez laquelle les choses se passent peut-Ctre autrement que dans l’espéce 
que nous avons soumise & l’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.{ 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 général” of the development of the gastrula of Echinoderms 
is more widely spread among Echinoderms than the following quotation 
* Op. cit., Pl. XI. fig. 9. 
+ Mem. Mus. Comp. Zool., Vol. TX. No. 2. 
t We are here brought face to face with one serious defect in Apostolides’ and 
Balfour’s observations, namely, the difficulty of knowing exactly the species which 
both studied. 
