636 H. GRAHAM CANNON 
6. Discussion. 
Kiihn has shown in his paper dealing with the development of 
the summer eges of Polyphemus (8) that the early develop- 
ment of Moina as described by Grobben (6) is very similar, 
in fact almost identical, to that of Polyphemus, and 
Samassa’s description (11) of a totally indeterminate method 
of development of the eggs of Moina receives no support 
from his work. These are the only two Cladocera in which 
a determinate type of development has as yet been described. 
Vollmer in his work on the resting eggs of the Cladocera 
(14) states that when the blastoderm consists of about two 
hundred cells a migration inwards takes place of about eight 
to ten of these from the future ventral side of the embryo. 
These multiply and form the genital rudiment. A similar 
proliferation of cells from a ventral blastozone later forms the 
‘untere Blatt’, and from this is subsequently separated a 
solid rod of cells which forms the gut. Because of the early 
separation of the genital rudiment Vollmer states that this 
method of development is intermediate between the deter- 
minate development of Polyphemus and Moina and 
the indeterminate type of development as deseribed by Agar 
(1) in Holopedium and Lebedinski (9) in Daphnia. 
A comparison of the genital rudiment as described by 
Vollmer for Daphnia with that of Simocephalus 
described in this paper shows certain differences. lirstly. 
the mode by which it passes into the interior of the embryo 
is different in the two cases. In Daphnia this is brought 
about by a few cells that wander from the blastoderm into 
the interior of the egg, presumably by the action of an inwardly 
directed cytotaxis. In Simocephalus, on the other hand, 
the group of cells forming the genital rudiment passes into 
the interior by an invagination and only becomes internal 
when the edges of the pit caused by this invagination have 
srown together and fused. Here again the invagination may 
be brought about by a similar force. However, the extent of the 
invagination varies considerably, sometimes the pit is very 
