EXPERIMENTAL EMBRYOLOGY 163 



Reith (1925) obtained similar results with the housefly. Geigy (1931a) 

 reports that killing the posterior pole of Drosophila eggs by ultraviolet, 

 irradiation during cleavage results in adults whose gonads are com- 

 posed of only mesodermal elements (i.e., contain no germ cells). Shorter 

 irradiation frequently resulted in unilateral castration. The single 

 gonad might be small, of normal size, or larger than normal. To explain 

 these large single gonads he accepts Ruppert's suggestion that in addi- 

 tion to killing some of the cells the ultraviolet rays cause an adhesive- 

 ness of the germ cells so that they stick together during migration into 

 the embryo instead of separating into two gonadal groups. More exact- 

 ing data on Drosophila are given by Rowland and Robertson (1934). 

 They dechorionated eggs and killed part or all of the pole cells by care- 

 fully localized point cauterization. The sole effect was partial or total 

 sterility. Therefore the pole cells are not only destined to form the 

 definitive germ cells but incapable of being regenerated by an otherwise 

 normal embryo. 



Order of Embryonic Determination. — In addition to the determina- 

 tion process passing anteriorly and posteriorly from the thoracic differ- 

 entiation center there is sometimes a later, secondary determination for 

 specific organ characteristics. The data are from intermechates. 



To explain intersexes of the gypsy moth, Goldschmidt (1927, 1931) 

 postulates that the individuals begin development as one sex, that a 

 physiological change constituting a turning point occurs, and that sub- 

 sequent development is characteristic of the other sex. All structures 

 finally determined before the time of this change will be of the former 

 sex; all determined later will be of the opposite sex. An intersex is then 

 a "time mosaic" of male and female parts due to differences in the time 

 of determination of specific organ types (Shull, 19306). In apphcation 

 it is assumed that in the induced change from one sex to the other the 

 order of determination is the reverse of the order of modification in 

 specimens successively more like the opposite sex. Applying this, 

 Goldschmidt finds that the sex of the gonads and abdomen is determined 

 before that of the wings and antennae. One of the most interesting points 

 is that the onset of histological differentiation of the sexual characters 

 does not necessarily signify that the sex of those organs is finally deter- 

 mined. This is clearly shown by the gonads. These may develop to the 

 point of containing almost mature eggs or sperm and then following sex 

 reversal have the differentiated germ cells degenerate and be replaced 

 by the differentiation of germ cells of the opposite sex. All this occurs 

 because of the genetic constitution of the cells themselves and presum- 

 ably is not influenced by hormones. 



Shull (19306, 1931) reports that when the offspring of winged females 

 of the aphid Macrosiphum are gradually changed from gamic to partheno- 



