viii ARTHROPOD A 261 



OTHER INSECTA 



The account which Wheeler gives of the development of the 

 beetle Doryphora bears out Hirschler's account of Donacia in every 

 important point. There are, however, several interesting features 

 in which Doryphora differs from Donacia. The embryonic area 

 extends so far on to the dorsal surface of the egg at its posterior end 

 that it covers two-thirds of it, consequently the embryonic area of 

 Doryphora encircles the egg in a longitudinal direction almost as 

 completely as does that of the spider. This posterior extension 

 occurs after amnion and serosa have been formed, and as the origin 

 of the posterior amniotic fold is forced farther and farther forward 

 on to the dorsal surface of the egg, by the growth in length of the 

 embryonic area, yolk passes in between its two limbs, i.e. between the 

 amnion and serosa. 



The serosa ultimately forms a complete envelope, its anterior and 

 posterior folds meeting one another as they do in the Vertebrate 

 embryo. It then separates from the egg completely and adheres 

 to the egg-shell, consequently, when reversion begins, only the 

 amnion is ruptured and only the remnant of the amnion forms the 

 secondary dorsal organ. The grub of Doryphora has three pairs of 

 larval eyes (and this is true of Donacia also); they arise as little 

 ectoderma-1 pits on the side of the head, in each of which the central 

 cell is larger and clearer than the others. 



But Wheeler (1889) has also studied the development of Blatta, a 

 cockroach and member of the order Orthoptera, a group of insects 

 which differ from- the Coleoptera inasmuch as they emerge from 

 the egg as perfect insects except for the want of wings. 



In the case of Blatta the embryonic area is always confined to 

 the ventral surface of the egg and is never as long as the egg. The 

 mode of formation of the primary layers differs from the mode 

 observed in Doryphora and Donacia in several important points. 

 There is no gastral groove, but a posterior blastoporal pit, the cells 

 lining which proliferate actively, giving rise to two streaks of 

 cells which extend forwards and constitute the lateral bands of 

 mesoderm. When the coelomic sacs make their appearance those 

 in the thorax extend into the legs as in Peripatus, and are not 

 confined to the bases of the leg as in Donacia. No trace of the 

 endoderm can be distinguished until a much later period. When 

 it is seen it consists of two thin bands of very small cells lying 

 on the lateral surface of the yolk and connected with sheets of 

 similar cells attached to the inner ends of stomodaeum and 

 proctodaeum. 



By this time, however, the mesoderm has become segmented and 

 has developed coelomic cavities, and the walls of these have under- 

 gone differentiation. 



From the similarity of the endoderm in Blatta at this stage to its 

 condition at an earlier stage in Doryphora, Wheeler concludes that 



