136 



PRINCIPLES OF EMBRYOLOGY 



when such eggs are fertilised by chromosomes bearing a Y chromosome 

 (which, if they could develop fully, would produce males). In this case 

 the alleviation is probably caused by supernumerary X-bearing sperm, 

 which enter the egg cytoplasm, but later degenerate, since they do not 

 unite with the egg nucleus; it is known that in Drosophila it is common 

 for five or six sperm to penetrate the egg. 



FortquX 



HinJqut 



Ntrv»ui susttm 



DIAGRAM OF MEDIAN LONGITUDINAL 

 SECTION OF FUSED EMBRYO AT 18 HOURS 



Figure 8.10 



Diagrammatic longitudinal section of Drosophila embryo developed from 



an egg oi a. fused mother mated to z fused male: displacement of organs due 



to faulty gastrulation movements. (From Counce 1955.) 



We are thus beginning to get, in Drosophila, some insight into the 

 interaction of genes and cytoplasm in early development, a subject which 

 is obviously of the greatest importance, but in which much remains to 

 be done. The study of hybrid merogons in Amphibia (p. 358) and echino- 

 derms belongs to the same general sphere of interest, but in those organ- 

 isms we cannot, as yet, investigate the effects of individual genes. 



3 . The transformation of the embryo into the adult. 



All insects, as tliey grow, pass through a series of moults, in which the 

 external cuticle is shed and a new cuticle formed around the enlarging 

 insect. As the moults proceed there are changes in the organisation of the 



