Mr. C. P. Sigertoos on the Pholadide. 237 
advanced stage of development. It is found but sparingly at 
Beaufort, so that my observations on this form were not 
extensive. However, one set of embryos which I secured 
were just at the end of segmentation and at the time of the 
derivation of germ-layers. In most of the eggs the large 
mesoblast was already formed ; but in one, more tardy than 
the others, it was derived from the right entoblast. ‘The two 
large entoblasts are of equal size. ‘The left in all cases 
divides first into two cells, the anterior slightly the smaller. 
The right soon follows, dividing like the left. Soon those of 
the left divide again, followed by division of the right, so that 
the entoblast consists of eight subequal cells, each quite high, 
bordering on the blastopore, and with the nucleus nearer the 
outer end, ‘The ciliation of the embryo is less perfect and 
developed later than in the preceding three species. It is 
flattened as in the other forms; but I could not observe with 
certainty the inwandering of the ectoblast-cells to form 
mesoblast. Soon after this” stage the embryos became 
irregular, 
The actual migration of ectoblast-cells to form part of the 
mesoblast, I believe, has not been described before. Lillie 
traces a “ larval mesoblast”’ to one of the ectoblasts of Unio. 
This migration of cells from the surface into the cleavage- 
cavity is present in great uniformity as to time and position 
of formation in the first three species I have described; and IL 
think we must believe that a like condition holds for 
T. navalis. 
This second origin of the mesoblast I take to be of double 
significance, in so far as it touches Hatschek’s description for 
fT. navalis (‘ Arbeiten Zool. Inst. Wien,’ vol. i. 1881). The 
early stages in the formation of the gut (the division of the 
entoblasts) are exactly like those of the other three forms— 
like all marine Lamellibranchs which have been accurately 
studied. At least during the early stages there is an invagt- 
nation of the entoblasts, and there is no evidence that small 
cells are formed from the single large entoblasts of either side, 
which later arrange themselves to torm the gut. Hatschek 
says he had no stages between those he represents in figs. 13 
and 14a. The embryos I had bridge this gap. His error 
in describing the formation of the gut arose, I believe, from 
his finding small cells lying on the entoderm in slightly later 
stages. ‘These, as I have shown, are mesodermal elements 
derived from the ectoderm which come to lie on the entoderm. 
I think the evidence is that the Pholadide in their early stages 
develop in the same manner, even to minute details. I see 
no reason to believe that Z. navalis departs from the other 
Ann. & Mag. N. Hist. Ser.6. Vol. xvi. 17 
