ENTOMOSTRACA OF MINNESOTA. 183 



that the differentiation has proceeded to the extent of producing the 

 greatest nnmberof 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 it& 

 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 D(fphn'm 

 schceferi is throughout striking as compared with Moina, the only one 

 of the DaphnldcB the development of which is fully studied. In the 

 summer eggs I have not been able to see the complete segmentation 



