W. E. Agar 305 



(in Daphnia) two at a time, one from each ovary. When ripe they are 

 likewise passed out of the oviducts into the brood pouch. 



Even when the eggs are at a very early stage of ovarian growth, 

 it is easy to determine through the transparent tissues of the animal 

 whether they are sexual or parthenogenetic. The two kinds of eggs 

 differ in colour and texture ; moreover, in the single sexual egg in each 

 ovary yolk deposition starts at the hind end and thence spreads forwards, 

 while in a parthenogenetic ovary yolk appears simultaneously in a 

 number of eggs spaced out along the length of the ovary. 



Copulation and fertilization in the Cladocera was long ago described 

 by Weismann (1880), and I can confirm his account in most particulars. 

 Briefly, if a female with sexual eggs in her ovaries is kept from copu- 

 lation, the eggs are not laid, but are retained in the ovaries till they 

 disintegrate. If copulation takes place, the eggs are laid into the brood 

 pouch, where they meet with the spermatozoa. If fertilized they become 

 enclosed in a special egg-case or "ephippium" secreted by the brood 

 pouch ; if not fertilized, they disintegrate in the brood pouch within a 

 few hours. 



During copulation the male introduces the tip of his abdomen into 

 the female brood pouch, and the spermatozoa are passed into it. Gener- 

 ally within a very few minutes after copulation the two eggs are laid 

 into the brood pouch. The stimulus of copulation seems to be almost 

 essential if oviposition is to take place. In the course of these experi- 

 ments I have had occasion to observe this particularly in the case of 

 more than 400 pairs of sexual eggs produced in the absence of males. 

 In only four of these cases were the eggs laid — in each case of course 

 disintegrating soon afterwards. Further proof that it is copulation which 

 induces oviposition is furnished by matings in which the male is a sterile 

 hybrid (to be described below). In all such cases the eggs were laid 

 shortly after copulation, though as the males contained no spermatozoa 

 the eggs could not be fertilized and disintegrated soon after laying. 



Eggs that have been laid after normal copulation meet with the 

 spermatozoa in the brood pouch and are fertilized. At first the eggs 

 are irregular in shape, but they soon round themselves off and take up 

 a position one behind the other in the middle axis of the body. The 

 walls of the brood pouch secrete the thick chitinous ephippium which 

 encloses the eggs and is cast off at the next ecdysis, which takes place 

 about 3 — 4 days after oviposition. The ephippium, containing its two 

 eggs, lies at the bottom of the water for an indefinite period. The 

 contained eggs may hatch in a few weeks, or not till after several years. 



