36 EMBRYOLOGY OF INSECTS AND MYBIAPODS 



potent. That they may play a part in the differentiation of the cortical layer of 

 cytoplasm during the cleavage period is highly improbable, since definite cj'^sto- 

 plasmic organization already exists before cleavage begins. 



Two general types of germ-cell differentiation in insects are illustrated 

 by Miastor and Droso-phila, respectively. In Miastor, as shown by the 

 studies of MetschnikofT (1866), Kahle (1908), and Hegner (1912, 1914), a 

 single primordial germ cell gives rise to all the germ cells of the organism. 

 In Droso-phila, on the other hand, a variable number of cleavage cells 

 migrate into the posterior polar plasm, and each is constricted off as a 

 primary germ cell (Huettner, 1923). In this case the germ cells are of 

 polynuclear origin. 



With those pterygotes in which the primordial germ cells are early 

 distinguishable, they are found in a little mass at the posterior end of the 

 egg (Fig. 313), a position from which they are pushed forward at the tip 

 of the caudal end of the developing embryo and into the interior. They 

 then migrate forward, perhaps passively, dividing into two groups, one 

 going to the right, the other to the left, until they reach their definitive 

 position on the mesodermic genital ridges which are to form the gonads. 

 In those pterygotes in which the germ cells are not recognizable as such 

 until they have reached their definitive position after the formation of 

 the coelomic sacs, it is possible that they have a similar early history, but 

 this has not been demonstrated because of their probable similarity to the 

 somatic cells. In the apterygote Isotoma the primordial germ cell is 

 differentiated either at the 16- or at the 32-cell stage and develops into a 

 cluster. This cluster divides into two groups which pass from the yolk 

 into the tissue of the visceral wall of the mesodermic somites of the 

 third and fourth segments. From here they migrate to their definitive 

 position in the first to the third segments. 



Although there are many families and some orders of Eutracheata in 

 which the time of the segregation of the germ cells is not known, never- 

 theless the data on a sufficient number are available to make an attempt 

 at generalization, adopting the following divisions of Nelsen (1934): 



1. In the Diptera, Siphonaptera, parasitic Hymenoptera, Isotoma, 

 and certain Coleoptera (some chrysomelid beetles and the curculionid 

 Calandra granaria) the germ cells are distinguishable as such some time 

 during the formation of the blastoderm. In the midge Chironomus the 

 primordial germ cell appears at the time of the second cleavage division; 

 in Isotoma it is 1 cell in the 16- or 32-cell stage; in Miastor metraloas and 

 Phytophaga destructor it is differentiated at the 8-cell stage. 



2. In the Dermaptera, some Homoptera (certain Aphidae), a lepi- 

 dopteran (Euvanessa), and certain Coleoptera (some chrysomelid beetles, 

 the curculionids Calandra callosa, C. coryzae, and Brachyrhinus ligustici) 

 the germ cells are apparent immediately after the blastoderm is formed. 



