. Biology in America. Biology. The Role of the Chromosomes 231 shown, for the fertilized egg becomes either a queen or a worker, according to the kind of food which it receives. In other parthenogenetic forms the sex relations are not so clear. Among the fresh water Crustacea the daphnids pre- sent a well-known example of parthenogenesis. When the daphnids appear in spring or summer only females are found. They lay thin-shelled eggs which develop without fertilization into other females which in turn lay parthenogenetic eggs, and so the story goes for several generations; when suddenly, males
. Biology in America. Biology. The Role of the Chromosomes 231 shown, for the fertilized egg becomes either a queen or a worker, according to the kind of food which it receives. In other parthenogenetic forms the sex relations are not so clear. Among the fresh water Crustacea the daphnids pre- sent a well-known example of parthenogenesis. When the daphnids appear in spring or summer only females are found. They lay thin-shelled eggs which develop without fertilization into other females which in turn lay parthenogenetic eggs, and so the story goes for several generations; when suddenly, males make their appearance, together with the "winter" or "resting" eggs, which are fertilized, and surrounded by a thick shell, in which they pass the winter inactive in the ooze at the bottom of the water. AVith the advent of warm weather the following year these eggs hatch, giving rise to. Molted Skin and Liberated Egg Case op a Daphnid Photo by Lloyd, from Needham and Lloyd's "Life of Waters," Comstoek Publishing Company. Inland parthenogenetic females, which repeat the story. The time of appearance of males varies in different species. In some it appears to be correlated with the lowered temperature of autumn, and in others with the drying up of the pools in which the animals live. A similar life history is presented by the rotifers, or "wheel animalcules," so called from the circlet of vibrating cilia at the anterior end. The group of nematodes or thread worms present some curious modifications of the sex cycle. Some of these are parasitic, others are free-living and yet others present a so-called "alternation of generations" in which one genera- tion lives parasitically and the other free. Moreover some are hermaphroditic, others bi-sexual. Among the bi-sexual, free- living forms, parthenogenesis is of common occurrence, having gone so far that in one species, males occur only in the ratio of 13 to 100,000 females, while in ot
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