26 PHYSIOLOGICAL GENETICS 



axial skeleton is shortened and ankylosed in the posterior region 

 (description in Du Toit, 1913; genetics in Dunn, 1926). The 



details of development have not yet been worked out, but we have 

 already mentioned the fact that this condition has also been 

 produced as a phenocopy by Danforth (1932) (see page 10). 



It is rather remarkable that the type of genie effect upon 

 development found in vertebrates may also occur in insects. 

 This means an early effect upon a more generalized develop- 

 mental process which leads to a number of later consequences. 

 Such facts were described by Speicher (1933) for the development 

 of the eye mutant Eyeless in the wasp Habrobracon. Here the 

 development proceeds regularly until a late larval stage. At this 

 time, a portion of one pair of imaginal disks fails to invaginate 

 sufficiently to become separated from the larval hypodermis. 

 This leads to a retardation of the development of the dorsal part 

 of the head capsule, resulting in the formation of large lobes in 

 that region. Later, all the elements of the eyes and head appear 

 in normal order but are modified in size and shape by the previous 

 event. 



A third group contains work in Drosophila, where some decisive 

 facts could be found in the work of Schultz (1935) on eye colors, 

 of Medvedew (1935) on eye form, of Goldschmidt (1935c, 1937) 

 on the mutants of the wing, and of a few others. As the last 

 eases seem to be the simplest for explanation, and as here the 

 parallel with the phenocopies has been established, we shall 

 describe them first. 



One of the phenocopies that may be most easily produced both 

 by heat and X rays are scalloped wings. Their type covers 

 exactly the phenotype of the vestigial allelomorphs nicked 

 (vg ni ), notched (vg no ), up to snipped (yg sn ). The sensitive period 

 is situated at the end of larval life. A study of the development 

 of the series of ygr-alleles gave remarkable results (Goldschmidt, 

 1935c, 1937). Phenotypically a wing of this series appears like a 

 complete wing from which parts have been removed starting at 

 the tip and proceeding toward the base of the wing until only a 

 small stump has been left in the alleles vestigial and No wing 

 (see Fig. 3). (This was known to the first observers but neglected 

 in later work.) The morphology of this series already indicates 

 that the typical subdivision of the wing surface with its veins 

 and spaces between the venation must have occurred in develop- 



