628 H. GRAHAM CANNON 
all the work done on determinate development in the Crustacea. 
Since then no further work has appeared on this subject, so 
that it would be mere repetition if that work were to be again 
summarized here. 
Vollmer (14), in his summary, states! ‘that these results 
peint to the fact that we must differentiate between two 
categories of eggs which possess different modes of develop- 
ment in relation to their yolk-content ; eggs poor in yolk 
show a determinate development with practically total seg- 
mentation, eggs rich in yolk show an indeterminate superficial 
type of development ’. Referring to his own work, he suggests 
that the developmental processes that he describes demonstrate 
an intermediate form between these two methods and make 
the transition less abrupt. Irom the work recorded in this 
paper it would seem that the development of the summer 
eggs of 5. vetulus shows, perhaps more markedly, an inter- 
mediate stage between the determinate and indeterminate 
methods of development of Cladoceran eggs. 
This work was carried out in Professor MacBride’s laboratory 
at the Imperial College of Science, and I must thank Professor 
MacBride for valuable suggestions and for kindly reading the 
manuscript. 
2. Msrnop. 
In all cases the embryos were dissected out of the brood- 
pouch into the smallest amount of water possible before being 
fixed. I ixing the whole Daphnid with the embryos still in 
the brood-pouch gave unsatisfactory results. | 
It was found necessary to employ different fixatives for the 
various stages of development. For the early stages no reliable 
method was found. Carnoy’s fluid (Ac. Ale. Chloroform) 
gave good results, but the difficulty experienced was the 
unreliability of the fixative. The egg is surrounded by a tough 
membrane and it is this that causes the trouble. It is never 
possible to say whether it will burst or not under the action 
of the fixing agent. In the segmenting egg, if the membrane 
' My translation. 
