134 PRINCIPLES OF EMBRYOLOGY 



infective transmission of pigment formation in guinea-pig skin (p. 397). 

 This reminds one strongly of the Differentiation Centre and its behaviour 

 as described by Haget in Leptinotarsa. 



Another method which has recently been employed is the application 

 of ultra-sonics for the purpose of stirring round the internal contents of 

 the egg (Selman and Counce, 1953, 1955). In cleavage stages this treat- 

 ment may cause many types of cytoplasmic disturbance. If it is applied 

 w^hen the nuclei have arrived at the surface, preparatory to forming a 

 cellular blastoderm, it may prevent the gastrulation movements, so that the 

 tissues differentiate fairly normally but in the positions which they have 

 before gastrulation occurs; or it may shift around the various cellular 

 regions, in which case the organs are later found in quite unusual positions. 

 In the latter case, there is usually some abnormaHty or deficiency in tissue 

 differentiation; in particular the hypodermis tends to be badly developed. 

 There are suggestions that some of these abnormaHties of differentiation 

 are consequences of disturbances of inductive relationships, but in most 

 cases the evidence is not clear. Selman and Counce find, however, that 

 gonads may develop in quite unusual parts of the body to which the 

 germ cells have been transported, and argue that these cells induce the 

 mesoderm near them to develop into the gonad sheath. (In Leptinotarsa 

 Haget [1952] finds that the gonad mesoderm can differentiate in the 

 absence of the germ-cells, and the same is probably true of Drosophila 

 [Aboim 1945] though here the evidence is not entirely convincing; 

 however even if it is accepted, this would not disprove the suggestion 

 that the germ-cells can also exert an inductive influence.) 



The most important aspect of the embryology of Diptera, however, 

 is not its analysis by the operative methods of normal experimental 

 embryology, but the opportunity provided by the enormous wealth of 

 genetical material available in Drosophila to study the action of genes in 

 early development. The pioneer in this work was Poulson (1945, 1950), 

 and the most recent work that of Ede (1954) and Counce (1955). The 

 principle which has been followed is to study the development in cases 

 where the genetic situation causes death before hatching (in which case 

 one can be certain that something fairly drastic is going on). A consider- 

 able number of factors have now been investigated, and the most import- 

 ant general points emerging seem to be the following. 



Genes may be active at a very early stage of development. Poulson 

 (1945) found that the absence of the whole or the greater part of the X 

 chromosome causes disturbances of the cleavages and the migration of the 

 cleavage nuclei. Ede found that certain sex-linked genes, which may be 

 point-mutations, or may be very small deficiencies, may have the same 



