THE INSECTS I35 



effect; thus it is not only large amounts of chromosome, but on the con- 

 trary individual genes which are active at this stage. 



A considerable number of genes are found to affect the process of 

 gastrulation, v^hich involves extensive movements of the blastoderm. 

 This is clearly a delicately balanced process, w^hich can easily go wrong; 

 it is an 'epigenetic crisis', that is, a time at which minor abnormalities 

 which have occurred earlier suddenly produce far-reaching and drastic 

 effects. 



Certain genes cause considerable abnormalities in tissue differentiation. 

 These seem to be of two kinds ; general retardation or impairment of 

 differentiation, the cells remaining rather embryonic in character; or the 

 switching of cells wliich should develop into one type of tissue into some 

 other of the characteristic types. An example of the latter effect is the 

 observation that in several cases an abnormally large proportion of the 

 hypodermis develops into neural tissue, leaving little or none to form skin. 

 This might be the result of a disturbance of an intra-dermal inductive 

 process within the hypoderm, of the kind postulated by Haget (p. 130). 



These facts make clear the importance of chromosomal genes in the 

 early developmental processes. Counce (1954) has recently studied in 

 detail some stocks in which the importance of the cytoplasm becomes 

 obvious. These are 'female-steriles', that is to say, races in which females 

 homozygous for a given gene produce eggs which do not develop 

 properly (see also Beatty 1949). This failure must be due to abnormahties 

 in the cytoplasm formed under the influence of these genes in the ovarian 

 tissues of the mother, hi one of the genes studied, deep orange, the 

 abnormality is similar to the first kind mentioned above, in that it is 

 manifested in the early cleavage divisions. Another, fused, affects some of 

 the elongation movements concerned in gastrulation (Fig. 8.10), while a 

 third brings about an arrest of differentiation during a certain period of 

 later embryogenesis. There is thus a considerable variety of processes 

 which such cytoplasmic factors may influence. 



The nature of the cytoplasmic factors is not at all clear. They may 

 perhaps be 'plasmagenes' (p. 387), that is to say, have some power of 

 reproduction; but it is unnecessary to make this assumption, since no 

 overall growth has occurred by the time they begin acting. A very inter- 

 esting fact, however, is that their adverse effects can be to a large extent 

 ' overcome by the normal allele of the locus. If eggs from a female homo- 

 zygous for one of these female-steriles (e.g. fused) is fertilised by sperm 

 carrying a normal X chromosome, complete development may occur, and 

 even if it does not do so, the egg develops more normally than otherwise. 

 Some alleviating effect of this kind, though less in degree, is found even 



