60 
fresh water Lamellibranchs also shows a great similarity be- 
tween them and the oyster. 
Flemming has given a very minute account of the segmen- 
tation of the egg of Anodonta (Studien in der Entwickelungs- 
.geschichte der Uajaden, von Walther Flemming in Prag. 
mit 4 Tafeln. Aus den lxxi. Bande der Sitzb. der k. Akad. der 
Wissensch. Ill Abth. Febr. Heft. 1875. Sitzung am 4. Feb- 
ruar, 1875), and his account of the process (pp. 38-58, and 
his figures, Taf. II. Figures 1-20), show that the segmentation 
is much like that of the oyster, except that the segmentation 
cavity makes its appearance very early. 
The segmentation of the egg of the Anodonta differs from 
that of the oyster egg in a number of minute details, but not 
so much so as to obscure the fundamental similarity. Flem- 
ming’s Figure 6, of Plate II, obviously represents the same 
stage as my Figure 13 ; his Figure 11 is the same as my Figure 
19; his Figure 13 the same as my Figure 22; his Figure 17 
the same as my Figure 26 ; his Figure 18 the same as my 
Figure 28, and his Figure 23 the same as my Figure 32. 
The polar globules are shown in most of his figures, and prove 
that, as in the oyster, the growing edge of the layer of ecto- 
derm is at the point which is farthest from these bodies. 
The early stages in the development of Unio have been fig- 
ured and described at length byPabl. (Ueber die Entwicke- 
lungsgeschichte die Malermuschel. Fine Anwendung der 
Xeimblatter — Theorie auf die Lamellibranchiaten, von Carl 
Xabl, Jenaische Zeitschr, X, 1876, 310-395; Taf. X— XII), 
and his account shows a still closer agreement with the oys- 
ter than is presented by Anodonta. 
The segmentation cavity of TTnio makes its appearance 
very early, but in other respects there are few differences be- 
tween his account and my own, except that I have not found 
in the oyster the two large cells which he says become pushed 
into the segmentation cavity of Unio, and give rise to the 
mesoderm. In a paper which is now in press, on the forma- 
tion of the digestive tract in the fresh- water Pulmonates, I 
have been compelled to call attention to the fact that certain 
figures in an earlier paper by Pabl, on the development of 
