52 TWELFTH ANNUAL KEPORT. 



consists, as the winter egg, of two sorts of yolk spheres. The 

 plasma or formative yolk contains 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 through- 

 out the ealier stages of the embryo; second, smaller globules of ap- 

 parently the same character, which are quite numerous and form a 

 very considerable part of the contents of the egg, In an egg of 

 about .35 mm. in diameter, the largest of the smaller size of oil 

 drops measured .029 mm. while the larger three exceeded .060 mm. 

 The oil drops are distinguishable by their light refractive 

 power, pellucidity and the intense dark brown or black color assum- 

 ed when treated with osmic acid. The latter reagent affects the 

 formative yolk but slightly. It will be seen that though the sum- 

 mer 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 

 schaefferi is throughout striking as compared with Moina, the only 

 one of the Daphnidae the development of which is fully studied. 

 In the summer eggs I have not been able to see the complete seg- 

 mentation described for Moina. The following stages are much 

 as described by Grobben. An invagination occurs and a median 

 swelling appears on the ventral aspect of the egg. 



Labrura and second antennae bud out and are soon followed by 

 the antennae, mandibles and two pairs of maxilla3, after which the 

 five pairs of feet soon appear. In an early stage there is present a 

 basal palpus to the second antennae, a fact not before observed, and 

 this persists as the small two-bristled wart found on the basal joint 

 of the antenna. It is a conspicuous object in the embryo and is 

 thus a true embryonic organ. 



The eyes of the embryo appear as two separate pigmented flecks 

 which approximate and are covered with an oval refractive body, 

 which later is penetrated by the pigment and divides to form the 

 small lenses. Soon after this the shell grows over the eye as de- 

 scribed for Moina. 



The first indication of the shell appears as two folds of the max- 

 illary region of the back, being thickest laterally. These grow 

 forward and backward to form the cephalic and body shield. At 

 a little later stage there appears a very interesting modification of 

 the shell which stands in close relation to the growth of the brood 

 sac. A slight protuberance appears on the margin of the shell in 



