65 
easily seen, although it is not at all complete or minute, but 
according to Brobetsky (Studien iiber die embryonale Ent- 
wickelung der Gasteropoden, von Dr. N. Brobetsky in Kiew. 
Arch. f. Mic. Anat. xiii. 1877, pp. 95-170. Taf. viii-xiii), the 
early stages in the development of Xassa are almost exactly 
the same as those of the oyster. The egg of Xassa has a 
large functional food-yolk, and the blastoderm which sur- 
rounds it is not simply an ectoderm, since it gives rise to all 
the germ layers ; but before the differentation of the spherules 
at the formative pole has made its appearance, segmentation 
takes place exactly as it does in the oyster, and the first ten 
figures of Brobetsky’s first piate might have been used, with- 
out the least change, to represent the stages of the oyster egg 
which I have given in my first nineteen figures. I hope to 
publish soon a short paper, illustrated by a comparative table 
of outline drawings of the segmenting eggs of various Mol- 
luscs, in order to illustrate my conception of their signifi- 
cance, but at present I must refer to the various original pa- 
pers. A reference to Brobetsky’s account and figures will 
show that his Eigure 1, Taf. YIII, is almost exactly like my 
Eigure 4 ; his Eigure 2 like my Eigure 5 ; his Eigure 3 like 
my Eigure 7 ; that his Figure 4 shows the change illustrated! 
more at length in my Figures 8, 9, 10 and 11 ; that his Figure 
5, A and B, shows the same stage as my Figure 13 ; that his- 
Figure 6 is the stage 15 of the oyster; his Figure 7 the stage 
16 ; and that his Figures 8, A and B, are the same as 111 / 
Figures 18 and 19. The sections of these stages which Bro- 
betsky gives in Elate IX, indicate that the early appearance 
of bilateral symmetry in Xassa and Urosalpinx is a condi- 
dition of things which has been brought about by the pres- 
ence of a large food-yolk, which does not undergo segmen- 
tation, and this conclusion is confirmed by a comparison with 
Brobetsky’s account of the development of Xatica and Fusus,, 
where a true food-yolk is lacking, and the embryo is radially 
symmetrical during the early stages. 
The facts which I have given in regard to the oyster show 
that the peculiar early stages of segmentation are of no func~ 
o 
