J. L. BROOKS 87 



to as the brood pouch or brood chamber, by the finger-like pro- 

 jections on the posterior part of the abdomen (see Fig. 1). The 

 number of eggs is influenced by the size and age of the female, 

 but for females of a given size, the level of the food supply is 

 the principal determinant of clutch size. While very large in- 

 dividuals may produce twenty, or thirty, or even up to fifty eggs 

 per brood, the smaller adults under the "usual" range of condi- 

 tions may have from none to five eggs per brood (Slobodkin, 

 1954; Brooks, 1946). These eggs develop directly into free-swim- 

 ming individuals which escape from the brood chamber during 

 the next molting of the parent. It should probably be noted 

 here that Daphnia embryos develop at the expense of the ma- 

 terial stored in the egg; there is no nourishment otherwise derived 

 from the mother while the embryos are in the brood chamber. 



Although this diploid parthenogenesis is repeated as long as 

 nutritive conditions remain adequate, a sudden drop in food 

 intake can, in some of the females, provoke a change in the 

 reproductive physiology. This change involves the cytoplasm of 

 the developing oocytes as well as the meiotic events of the nu- 

 cleus and the structure of the developing carapace. The cyto- 

 plasm of the oocytes becomes dark brown and opaque, as op- 

 posed to the light-colored (green-gray-blue), hyaline appearance 

 characteristic of parthogenetic eggs. The nucleus undergoes both 

 reduction divisions so that the resulting egg is haploid. These 

 eggs must be fertilized before they will develop. The cells of 

 the dorsal part of the carapace change from flattened and color- 

 less to columnar and pigmented, and form the ephippium (the 

 saddle-shaped area apparent on one female in Fig. 1 ) . The ephip- 

 pium clasps the two fertilized eggs which have been extruded 

 into the brood chamber. The eggs undergo partial development 

 while in the brood chamber, but soon reach a quiescent stage. In 

 this dormant state the eggs can withstand freezing or drying. At 

 the time of the next molt the ephippium, clasping its eggs, is 

 shed with the rest of the exoskeleton. It may sink to the bottom 

 or be caught in the surface film. Its ability to stay in the surface 

 film is a consequence of the hydrofuge nature of its cuticle. 



After the female has shed her old exoskeleton with its ephip- 



