OLIGONEPHRIDIA 259 



and related plants. The germ band forms on the posterior half on 

 the concave side of the reniform egg. The caudal end then pushes 

 into the yolk, and gradually the entire embryo becomes wholly immersed. 

 The head, somewhat curved up but located not far from the posterior end 

 of the egg, moves toward the surface, pushing the amnion against the 

 serosa. At this point a rupture occurs. The subsequent behavior of 

 the embryo in rotating into its original position of head foremost while 

 emerging from the embryonic envelopes does not differ in essential fea- 

 tures from that of the Homoptera and numerous other insects. Shortly 

 before the fusion of the amnion and serosa, and before revolution, a rup- 

 ture of the chorion occurs at the cephalic end through which the nymph 

 later emerges. Here we have another instance of a break in the eggshell 

 occurring long before hatching of the insect. 



HOMOPTERA, APHIDIDAE 



The Egg of the Viviparous Parthenogenetic Generations. — The Aphid- 

 idae have a type of development known as "heterogamy," or cychc repro- 

 duction. This is characterized by an alternation of parthenogenetic 

 generations with a sexual generation. In the north where the winter 

 interrupts the production of young or in hot climates where there is a dry 

 season, a single egg is laid by the female of the sexual generation which 

 tides over an unfavorable period. In the north in the spring there 

 hatches from the overwintering egg a female, known as the "stem 

 mother," which will parthenogenetically and viviparously produce young 

 females. When the young have become mature, they in turn will produce 

 parthenogenetic viviparous females, and so on for several generations. 

 For many species these are wingless except for a spring and a full migrant 

 generation which are winged parthenogenetic viviparous females. The 

 last of the parthenogenetic generations of the season will give birth to true 

 sexual forms, male and female. After mating, the female wih deposit 

 the single overwintering egg, completing the cycle. 



The eggs of the viviparous generations undergo their complete embry- 

 onic development in the vitellarium of the ovariole (egg tube). These 

 eggs have a scanty yolk, and they lack a chorion, the follicle of the ovarian 

 tube taking its place. Development is rapid; partially developed young 

 may be found in the ovariole before the mother is mature. 



That the embryonic development of various species of aphids differs 

 only in minor particulars has been shown by Toth in his comparative 

 study of the parthenogenetic generations of 28 species representing 19 

 genera. The early phases of oogenesis in parthenogenetic aphids have 

 been studied by Will (1883), Blochmann (1887), Stevens (1905), Tan- 

 reuther (1907), Hirschler (1912), Toth (1933), and others, Blochmann 

 first pointing out that the parthenogenetic eggs extrude only a single polar 



