64 
ermuschel, pp. 388-345), and concludes that the first is an 
adaptational modification of the last, which gives the Lamelli- 
branch an advantage in the struggle for existence, and which 
has therefore been preserved by natural selection. He says, p. 
244: “Dieinequale Furchung dem sich entwdckelnden Embryo 
einen Vortheil gewahrt, und dass dieser Vortheil um so 
grosser ist, je fruhzeitiger sich eine Hngleichheit in den Fur- 
chungsproducten bemerkbar maeht.” 
I think, however, that all the evidence which I have given 
points towards the conclusion that the peculiar segmenta- 
tion of the Lamellibranchs is due rather to the retention of 
characteristics which were adapted to some past condition of 
things than to direct adaptation to the present conditions of 
life ; and we must therefore look for its origin somewhere else 
than in the regular radially symmetrical segmentation of the 
small simple eggs of the Pulmonates. I think that a com- 
parison of my account and figures with the figures and de- 
scription given by Brobetsky and myself of the segmentation 
of the egg in those Prosobranchs where the eggs are few in 
number, large, and contain a large food-yolk which is of 
physiological importance, will fully support the conclusion 
that we have here the ancestral form of segmentation, which 
is retained by the small eggs of the Lamellibranchs. 
In a paper entitled “ Preliminary Observations on the De- 
velopment of the Marine Prosobranchiate Gasteropods,” (Ches- 
apeake Zoological Laboratory, Scientific Results of the Session 
of 1878, p. 121), I have given outline figures of a few of the 
stages in the segmentation of the egg of Urosalpinx, Plate 8, 
Figures 1, 2, 3, 4, fi and 10, and a reference to these figures 
will show that there is considerable resemblance between this 
and the oyster egg. A few small transparent micromeres 
separate off' from the surface of the large food yolk, which 
occupies the nutritive pole of the egg, and gives rise to a 
blastoderm which spreads over the surface of the food-yolk ; 
growing at one edge, partly by the division of the micromeres 
and partly by separation of new ones from the yolk. 
From the beginning of segmentation the egg is bilaterally 
symmetrical, and the general resemblance to the oyster egg is. 
