Nov2:|) J77CRODEUTOPUS GRYLLOTALPA COSTA. 325 
found to be the case in Mysis. In Microdeutopus I find no 
evidence of a teloblastic growth. The mesoderm cells seen in 
optical section in the stained and cleared egg are irregularly 
scattered under the ventral plate. 
Morphologically, the isopods and amphipods are very closely 
allied, and it is, therefore, all the more interesting to note how 
similar are their modes of development. If we compare a dia- 
gram of Jaera, after the germ layers have been laid down, with 
one of Microdeutopus, somewhat before the entodermal invagi- 
nation has taken place, the similarity will at once become 
apparent (see diagrams). 
Heider (91), in writing upon the formation of the isopod 
embryo, states that the dorsal cells, as the embryo develops, 
become pushed together, and these cells later undergo degene- 
ration. On page 352 he adds: “ Es ist die Méglichkeit, die wir 
oben andeuteten nicht ausgeschlossen, dass in dem Dorsalorgan 
bloss die Involutionsform des Nahrungsdotter bedeckenden 
Blastodermtheils vorliegt. Die Involution wurde sich dann 
bei den Amphipoden-Typus durch Einstiilpung, bei den Onis- 
cus-Typus durch Amputation einleiten.” If such is the case, 
then in both forms we have a ventral plate covering the ven- 
tral pole, at the posterior end of which the entoderm arises. 
Behind the entoderm lie the vitellophags or the dorsal cells 
in the amphipods, the similarity between these has been pointed 
out, and beyond the vitellophags the dorsal cells of the isopods 
or the dorsal organ. of the amphipods, according to Heider. 
