1885.] 



CLADOCERA. 



11 



much more numerous in the outer capsule, lying crowded together 

 in several layers, whereas in the inner capsule (see fig. 5) only 

 a single layer of considerably larger cells can be observed. 



The ova contained in the ephippia (see figs. 3. 6) are of a 

 regnlar oblongo-ovate form, and measure 0,03 mm in length. 

 They exhibit a very strong, but elastic and perfectly translncent, 

 egg-shell (chorion), quite structureless and of a smooth and 

 lnstrous appearance, conspicuously contrasting with the dull aspect 

 of the surrounding parts (see fig. 3). The yolk-mass is of a light 

 bluish green, and throughout uniformly and finely granulous 

 without exhibiting any distinct oil-globules, as is the case in the 

 so-called summer-eggs. 



Respecting the formation and further development of the 

 embryo, no essential difference from that known in the summer- 

 germs can be detected, as appears on comparing the figures 

 (6-9) here annexed. Nevertheless, a very marked peculiarity 

 in the development of the winter-eggs may be stated, viz., the 

 fact that the chorion, or egg-shell, is retained during the whole 

 embryonal development, while in the summer-germs, as is well 

 known, the exceedingly thin egg-membrane very soon, and long 

 before the embryo has attained its full development, is east off 

 and found in the ,,matrix" or incubatory cavity, of the parent 

 animal, along with the embryos, as minute, curly cuticles. 



During the progress of development the ova increase rapidly 

 in volume, attaining in the last stage (fig. 9) almost double 

 their original size. Owing to this increase, the valves of the 

 ephippium, together with those of the inner capsule, become 

 gradually forced apart, opening along the ventral side, so as 

 to expose a part of the enclosed ova. I have often found 

 the development to proceed unequally in the two eggs, and, 

 when such is the case, the ephippium assumes a somewhat irreg- 

 ular form, appearing much more tumid at the one extremity 

 (generally the hinder) than at the other. When the embryo has 

 attained its full development, the egg-shell becomes very thin 

 and finally bursts, whereupon the embryo drops off into the water 

 through the widely gaping valves of the ephippium, generally with 



