326 LANGENBECK. [Vou. XIV. 
The great difference lies in the origin of the mesoderm, and 
more work will have to be done upon other amphipods before 
this can be decided. It, however, is a great question to my 
mind if the dorsal cells in the isopods and the dorsal organ of 
the amphipods can be homologized. Heider himself merely 
suggests the possibility, as his conclusion was drawn solely 
from the work of Bobretzsky ('74). 
If we compare embryos of the later stages we find that in 
both cases the embryo is folded dorsally (see diagram and Figs. 
2.0 
Beg JL8 
J-d.g 
a. 
36, 37). In the amphipod, however, when the cells which 
will form the dorsum of the animal develop, the least resist- 
ance to the pressure exerted through their growth seems to 
be on the ventral pole, and, therefore, the embryo folds over 
ventrally (Figs. 38 and 42), whereas in the isopods the least 
resistance is in the dorsal region. Might not the different 
mode of folding in the two cases be due to the fact that by the 
time the cells which will form the dorsum of the animal de- 
velop the ventral pole in the isopods is further differentiated 
than is the case in the amphipod, and, therefore, offers greater 
resistance to the pressure exerted upon it by the growing 
region? 
Formation of the Liver Tubes and the Intestine. 
As has been described above, the entoderm arises as a true 
invagination. After the closure of the blastopore the cells 
migrate into the yolk area as an irregular mass, and by amoe- 
boid motion migrate to both sides of the body in much the 
