54: 
ment of Unio (Entwickelungsgeschichte der Malermuschel, , 
Figures 28 to 32), which is very similar to this, both in out- 
line and in internal structure, and Flemming has figured a 
very similar stage in Anodonta (Entwickelungsgeschichte der 
FTajaden, Taf. II Figure 32), but each of these authors re- 
gards the surface where the polar globule is placed as poste- 
rior. The published accounts of the transformation of the 
glochidium into the adult Unio or Anodonta (Ueber die post- 
embryonale Entwickelung unserer Susswassermuscheln (Ano- 
donta). von Dr. M. Braun in Wurzburg, and : Zur Entwicke- 
lungsgeschichte der Teich- und Flussmuschel. von Carl 
Schierholz.) are not sufficiently explicit to decide what the 
relation between the body of the larva and that of the adult 
really is, and until some one publishes a satisfactory illus- 
trated account of the transformation, the fact that the velum 
of the oyster certainly makes its appearance at the point 
which is occupied by the polar globule must lead us to believe 
that Flemming and Babl are in error, and that the region 
between the letters a and i of Babl ? s Figure 28, is that 
which is occupied by the velum in the the marine Lamelli- 
branchs, and therefore the anterior. 
The free-swimming stage may be reached, under excep- 
tionally favorable circumstances, within two hours after im- 
pregnation, but it is not usual for the embryo to attain to this 
degree of development in less than twenty-four hours, and it 
may require more than two days to reach it. The duration 
of this stage also varies greatly, but after from one to twelve 
hours the embryo will be found to have assumed the form 
shown in Figure 36, which is the same view as Figure 32. 
The outline of the body has not undergone much change, and 
the anterior end is still rounded and carries the velum, while 
the pointed anal papilla, a , occupies the posterior end. The 
foot-like protuberance on the ventral surface has disappeared, 
and the blastopore on the dorsal surface has entirely closed, and 
the ectoderm has become continuous over it, thus leaving the 
endoderm as a spherical body of cells inside the body cavity. 
I was not able to discover any central cavity inside this mass, . 
but the cells are so opaque that it would! be very difficult to > 
