59 
kongl. Yetenskaps-Akademiens Handlinger. Stockh. 1848)*. 
contains nearly all the present knowledge of the process of™ 
segmentation in the marine Lamellibranchs. I have not the- 
paper before me as I write, and my notes upon it, which were 
made several years ago, do not contain any figures, so I am 
not able to make a minute comparison, but the figures which 
he gives of the segmenting eggs of Crenella and Cardium are 
essentially like the same stages in the development of the 
oyster egg, and show that in these two genera the egg divides- 
into a single macromere situated at the nutritive pole, and a 
number of smaller mieromeres situated at the formative pole, 
and that the macromere is gradually surrounded by a layer of 
ectoderm cells, which are formed in part by the division of 
the micromere and in part by the separation of new micro- 
meres from the macromere. 
According to the short abstract which Brobetsky (Studien 
iiber die Embryonale Entwickelung der Gasteropoden, von 
Dr. 1ST. Brobetsky aus Kiew. Arch. f. Mik. Anat. 1870, pp. 95 — 
Taf. viii-xiii) gives of Loven’s observations upon the segmen- 
tation of the egg of Crenella, p. 104, the resemblance to the 
oyster does not stop here, but extends to more minute details. 
The egg divides, as it does in the oyster, into a first and a 
second micromere, and a macromere which is more trans- 
parent than the mieromeres. After these three spherules have 
become distinct, one of the mieromeres fuses with the macro- 
mere, and all traces of it disappear for a time. The other then 
flattens down upon the compound mass, and the egg assumes 
the condition shown in Figure 13. Soon both the first and 
the second mieromeres again become prominent, and then 
divide, so that there are now four mieromeres at one pole of 
the egg and one larger macromere at the other. One of the 
four now fuses with the macromere again precisely as it does- 
in the oyster, and a stage like my Figure 19 is reached. As 
in the oyster, Loven notices the alternation of periods of rest 
and activity for some time longer, and the agreement with 
the oyster appears to be most complete. 
Our knowledge of the early stages in the development of the 
