132 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



It seems evident that twinning is just one step in advance of a very 

 highly speciaHzed embryological metamorphosis exhibited by some of the 

 parasitic Hymenoptera that develop in the egg stage of their hosts. This 

 is demonstrated by the origin, presence, and similar function of the 

 trophamnion in both the monembryonic and the polyembryonic species. 

 The close relationship between monembryonic and polyembryonic insects 

 with regard to the origin of polyembryony is exhibited in three species of 

 Platygaster: herricki, hiemalis, and vernalis. All are parasites of the 

 Hessian wheat fly and oviposit in the egg of the host. All species occur 

 in the same locality. P. herricki develops only by monembryony; P. 

 hiemalis develops both monembryonically and by the twinning process, 

 the female depositing from four to eight eggs in the host egg which give 

 rise to about the maximum number that the host larva can tolerate. 

 P. vernalis develops only by polyembryony in its generalized form, 

 depositing one egg that gives rise to 15 to 20 adults, the maximum number 

 that the host larva can accommodate. 



A number of references to the presence of a cellular envelope about the 

 embryo of monembryonic entomophagous parasites have been published. 

 Some authors, in speaking of the embryology of the parasitic Hymenop- 

 tera, term this envelope the "amnion"; others speak of it as the "serosa"; 

 still others consider that both amniotic and serosal layers are present ; and 

 finally the terms "pseudoserosa" or "trophic membrane" or "tro- 

 phamnion," in part derived from the terminology used in polyembryony, 

 are applied. In this group of insects the cells constituting the envelope 

 apparently serve a nutritive function. Jackson (1928) and, recently 

 and more briefly, Vance (1931) have reviewed this subject. 



PARTHENOGENESIS 



The natural development of an egg without having been fertilized 

 by a male gamete is the phenomenon known as "parthenogenesis." It 

 occurs naturally in several groups of animals, among them rotifers, 

 crustaceans, and insects, and may artificially be produced in echinoderms, 

 mollusks, and amphibians. Since parthenogenesis is of interest to the 

 embryologist chiefly because of its cytological features in connection with 

 maturation, only a brief survey of its general aspects need be given here. 



The various types of parthenogenesis may be classified (A) in accord- 

 ance with the sex of the parthenogeneticall}^ developing individual or 



(B) according to the regularity of the occurrence of the phenomenon or 



(C) with respect to the chromosome behavior of the individual. 



A. In accordance with the sex: 



1. Arrhenotoky, when unfertihzed eggs give rise to males, fertilized 

 eggs to females. 



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