THE INSECTS 



133 



blastoderm stage, the position of the centre is fixed, but a small amoimt 

 of regulation is still possible within individual organs; that is to say, if an 

 egg in this stage is ligatured, each end forms only part of an embryo, but 

 each of these parts consists of a certain assemblage of complete regions, 

 i.e. a complete anterior head, or complete jaw region, complete thorax 

 or complete abdomen. At a slightly later stage, even this degree of 

 regulatory power is lost, and ligatured eggs develop in a completely mosaic 

 way, producing part thoraces or part abdomens exactly according to the 

 location of the ligature. 



In fully determinate eggs, such as those of Diptera, almost com- 

 pletely mosaic development occurs from the very earliest stages, and 

 after cauterisations or ligaturing, no sign of regulation is found. Dwarf 

 embryos have, however, been produced by strong centrifuging which 

 shifts the cortex (Padi 1927) and it appears that it is in the cortex that the 

 forerunners of the embryonic organs are located. Small drops of the 

 internal cytoplasm can in fact be removed (by pricking and allowing 

 them to exude) without causing abnormahties in development (Howland 

 and Sonnenbhck 1936) (Fig. 8.9). 



FlGUHE 8.9 



a, Normal embryo of the dipteran fly Calliphora; b, embryo formed after 



constriction of the egg at the i6-cell stage, showing no sign of regulation ; 



c, shortened and partially regulated embryo following strong centrifugation 



of newly fertilised egg. (After PauU 1927.) 



A little information on the epigenetics of the dipteran Drosophila, which 

 is so important for genetical studies, is beginning to be produced by less 

 conventional methods. Thus Yao (1950) has investigated histochemically 

 the distribution of acid and alkaline phosphatase, and finds that the latter 

 makes its appearance in the future thoracic region of the germ-band 

 some time after the mesoderm has invaginated, and gradually spreads 

 throughout the ectoderm, by a process which he compares with the 



