1895.] Embryology. 187 
means to him identity of origin in time and space, and he believes that 
two systems with like fates, but dissimilar origins, are not homologues. 
The protendoderm of Arthropods is a mesenchymatous tissue arising 
by migration, as in a planula of the hydrozoa. That of other ccelomates 
is epithelial from the start, and arises by gastrulation. Hence, in 
origin and character the two are essentially different. 
“The investigations of many observers on the development of 
sponges and hydroids have shown that in the ecelenterates the germ 
layers may be formed by other processes than gastrulation. ‘To-day 
it seems to M. Roule impossible to consider the germ layers of the meta- 
zoa as homologous. They differ from one another in origin. The pro- 
tendoderm (mesentoderm) of Arthropods does not correspond to that 
of other Ceelomates; but among Arthropods it is homologous, and 
among other ccelomates it is homologous. The difference between the 
lecithal planula of Arthropods and similar ones of other ccelomates lies 
in the origin of the endoderm in the latter by a true gastrulation. 
. H. Me. E. Kyower. 
