EMBRYOLOGY OF COCCIDS 117 



into germ cells. Later these migrate singly into the fourth to 

 the eighth abdominal segment and there become the forerunner 

 of the ovarian cells. 



The primitive germ cells in Forficula auricularis were also 

 found by Heymons ('95) at about the same stage as in the case 

 of the butterflies mentioned above. 



Heymons ('96), who could not find the genital organs in the 

 embryos of certain dragonflies and jVIay-flies, is of the opinion 

 that in the Ephemeridae and Odonata the first genital rudiments 

 seem to appear during the larval life. This is, indeed, a case of 

 the latest segregation of the germ cells so far recorded among the 

 insects. 



From the foregoing brief survey of the germ-cell formation in 

 insects, it becomes clear that a considerable variation is evident 

 in the manner and also in the time of their differentiation. This 

 variation is not a continuous one, but is such that the insects, 

 whose germ cell formation so far has been studied, can be grouped 

 in the following three categories : 



1. Cleavage differentiation: Diptera, possibly Hymenoptera. 



2. Blastodermic differentiation: Aphids, Chrysomelids, Lepi- 

 doptera and Coccids. 



3. Mesodermic differentiation: Orthoptera, Aptera, Neurop- 

 tera, Hymenoptera, parasitic forms, Ephemeridae, Odonata, 

 Dermaptera and probably Heteroptera. 



It is interesting to note that in all insects, the germ cells of 

 which have not been observed prior to the formation of mesoderm, 

 no special posterior inclusions have been found. Conversely, 

 in all the eggs having a certain kind of polar inclusion, whether 

 it be of nurse cell, epithelial cell or of nucleolar origin, the earliest 

 segregation of the germ cell has been recorded. Nevertheless, it 

 must be stated that the differentiation of the germ cells as such 

 cannot solely be accredited to the so-called 'Keimbahn deter- 

 minants' of Hegner. The germ cells of the Chrysomelid beetles 

 that possess the polar granules, appear at about the same time 

 as those of aphids and scale insects where there are no such 

 granules, but parasitic organisms. Such a striking case is that 

 of Euvanessa, where there is no possible inclusion of any sort 



