VIII 



ARTHROrODA 



263 



and the unsegmented mesoderm in the abdominal region. When 

 the division of the embryonic area into segments takes place, they 

 are found in segments 2-7 of the abdomen, in the coelomic wall in 

 the region of the genital ridge, as in Donacia. To them are added 

 modified coelomic cells, which we may term secondary germ cells, and, 

 together with the primary germ cells, they form a continuous mass 

 on each- side, which is the rudiment of the genital organ ; they are at 

 first found between contiguous coelomic sacs as well as in their walls. 

 When the coelomic sacs become choked up by the development of the 

 fat body, the uppermost section of each retains a slit-like lumen, the 

 inner wall of which is formed partly by the genital mass, the outer by 

 the layer of cells which gives rise to the pericardial septum. Above 

 the genital mass the inner wall is formed by a thin sheet of cells, the 

 so-called filament plate (fil.p, Figs. 208 and 209). Later^the slit-like 



FICJ. 207. Longitudinal section through the embryonic area of an embryo of 

 Phyllodromia germanica. (After Heynions.) 



coe, coelomic sac ; j/ 1 , germ cell produced before the coelom is dislodged ; g 2 , germ cell 

 formed from the wall of the coelomic sac. 



lumen disappears entirely, and this sheet breaks up into eight 

 vertical threads, united above by a longitudinal thread ; these are 

 the terminal threads of the ovarian tubes. Soon the genital mass 

 also becomes divided into vertical strings connected by a longitudinal 

 piece below. The eight vertical strings form the eight ovarian tubes 

 and the piece connecting them gives rise to the common oviduct on 

 each side. In the larval stage there are two oviducts. When the 

 adult condition is attained, a median oviduct is formed as an ectodermal 

 invagination, and into this the two oviducts of the larval period open. 



When we survey the rest of the Insect world, we hud that the 

 most interesting divergences from the types we have just studied are 

 presented by the Apterous insects whose development has been 

 studied by Heymons (1897 and 1905). By common consent these 

 insects are placed at the bottom of the class to which they belong. 



The development of Lepisma shows that the early stages of 

 segmentation, etc., resemble those of Donacia; there is the same 

 multiplication of nuclei and the same migration of these nuclei to 



