THE INSECTS 



125 



developed an embryo, and the two twins must at first have lain back to 

 back. As each spread out from the original ventral rudiment to close in 

 and cover their dorsal sides, the larger engulfed the smaller and, moreover 

 forced it to roll up the v^nrong side out. Thus the egg fmished by containing 

 a large embryo which enclosed a smaller twin which was inside-out, 

 surely one of the most remarkable arrangements ever produced (Fig. 8.3). 

 More insight into the causal processes of development is gained from 

 experiments in which the egg is divided in two transversely. This can 

 be done either by killing off one end by burning with a microcautery, or 

 better by constricting the egg with a loop of hair so that it is divided into 

 two parts. By using these methods, Seidel showed that during die dis- 

 persal of the cleavage nuclei there is a small region at the posterior end 

 which is essential for all further development, since if more than a very 



Figure 8.3 



An internal twin in PlatYcnemis, following a longitudinal split in. the egg 

 during the cleavage stages. Both longitudinal halves produced an embryo, 

 and as the larger germ-band grew round dorsally to enclose the yolk (direc- 

 tion of thick arrows) it reversed the dorsal closure of the smaller one (which 

 should have grown round in the direction of the thin arrows). In {a) the 

 internal embryo is shown dotted. In the transverse section (fc) note that the 

 internal embryo is inside out, with the appendages {Ap. i) inside and the 

 nerve-cord {Nc. i) outside the hypodermis {Hy. i) ; the organs of the external 

 embryo are at Ap. 2, Nc. 2 and Hy. 2. (After Seidel 1936.) 



small fraction of this end is completely removed, the egg forms a blasto- 

 derm but never proceeds to develop a germ-band or embryo. This 

 essential posterior region was named the 'formation centre' (BUdungs- 

 zentrum' in German). Its activity is almost certainly concerned with the 

 production of a diffusible chemical, since if the egg is constricted by a hair 

 which is not pulled completely tight but leaves a small channel through 



