R. C. VON BORSTEL 185 



meiosis or does there exist some extrinsic factor produced by the 

 female such as that described by Humphries (1955) in frogs? 

 Humphries' discovery that a low frequency of the frog eggs pre- 

 vented from going down a tied-off oviduct would pass through 

 meiosis uninhibited suggests that a substance secreted by the 

 oviduct causes the block. The alternative hypothesis is still pos- 

 sible although it is less likely that inhibition is intrinsic and does 

 occur; something in the coelom then activates the frog eggs. The 

 possibility that the inhibiting mechanism in the case of the 

 Habrobracon egg is built into the egg is more attractive than 

 extrinsic control, but no evidence that can help in solving this 

 problem is yet available. 



Karyokinesis 



In the consideration of the stage of development between one 

 nucleus prepared for cleavage and several thousand nuclei, three 

 items will be examined briefly: (1) prevention of cleavage of ac- 

 cessory sperm, (2) mitosis in broken eggs and egg exudates, and 

 (3) feulgen-negative nuclei in irradiated eggs. 



In embryogeny proper, of utmost interest is the problem of 

 nondivision of accessory sperm. Rothschild (1954) has briefly 

 considered prevention of polyandrous syngamy (Type II Inhibi- 

 tion of Polyspermy) in marine eggs and vertebrates, but he had 

 very little to say about insects. The accessory sperm in insects do 

 not cleave independently of the zygote nucleus. That sperm 

 nuclei are capable of cleavage, however, has been demonstrated 

 in Habrobracon by A. R. Whiting (1948), who obtained andro- 

 genetic male offspring from fertilization of heavily iiradiated 

 eggs. In these embryos, the egg pronucleus was held back by 

 chromosome bridges and was unable to fuse with the sperm 

 nucleus; the sperm nucleus cleaved normally. Polyspermy is 

 unusual but not rare in Habrobracon. It occurs in approximately 

 1 % of the eggs ( Speicher, 1936 ) . Gynandromorphs are much less 

 frequent than would be expected if all nuclei in dispermic eggs 

 normally developed. P. W. Whiting (1943a) discovered a strain 

 of Habrobracon that normally produced gynandromorphs with 

 androgenetic parts; Rothenbuhler et al. (1952) discovered a simi- 



