ENTOMOSTRACA OF MINNESOTA. 183 
that the differentiation has proceeded to the extent of producing the 
greatest number of protective layers without materially increasing the 
complexity, and thus the sensitiveness, of the organism. Under favor- 
able circumstances the development proceeds farther and near one 
pole appears a slight indenture of the surface which grows deeper and 
seems to form a true invagination. This blastopore, if such it really 
be, remains for some time, generally till the two ‘‘Scheitel’’ plates 
appear. These ‘‘Scheitelplatte’’ are formed by a simultaneous thick- 
ening and lengthening of the cells of limited areas on opposite sides of 
the egg, near the opposite pole from that occupied by the blastopore. 
The ‘‘Scheitelplatte’”’ are situated at right angles to a plane perpen- 
dicular to the blastopore. The nuclei of the cells of the ‘‘Scheitel- 
platte’’ are nearly 0.0208 mm. in diameter, while those of the other 
blastoderm cells are about half that size. : 
The egg remains a long time in this stage, while the following 
stages are passed through quite rapidly till the embryo assumes its 
nauplius form. The remainder of the development agrees, so far as 
seen, quite fully with that of the summer eggs, to which we will now 
return. 
The summer eggs vary greatly in size and number, but are nearly 
as large as the winter eggs. The number is sometimes reduced to 
two or three or rises to as many as fifteen or even more. In color the 
eggs vary from green to brown. The fresh egg consists, as the winter 
egg, of two sorts of yolk spheres. The plasma or formative yolk con- 
tains colored globules of rather small size, distributed throughout the 
whole of the mass quite uniformly. The food yolk or oil globules 
assort themselves in two sizes; first, a few (generally three) very large 
oil drops, which persist throughout the earlier stages of the embryo; 
second, smaller globules of apparently the same character, which are 
quite numerous and form a very considerable part of the contents of 
the egg. In an egg of about 0.35 mm. in diameter, the largest of the 
smaller size of oil drops measured 0.029 mm., while the larger three 
exceeded 0.060 mm. ‘The oil drops are distinguishable by their light 
refractive power, pellucidity and the intense dark brown or black 
color assumed when treated with osmic acid. The latter reagent 
affects the formative yolk but slightly. It will be seen that though 
the summer egg is nearly as large as the ‘‘Dauerei’’ in some cases, yet 
the relative amount of formative yolk is more diverse than at first 
appears. 
The great similarity between the two sorts of eggs in Daphnia 
schefferi is throughout striking as compared with Moina, the only one 
of the Daphnide the development of which is fully studied. In the 
summer eggs I have not been able to see the complete segmentation 
