146 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



6. Centrifuging the egg in various positions. 



7. Modification of environmental factors. 



8. Genetic analysis of "normal" mosaics, intersexes, and gynandro- 

 morphs in studies on fertilization, cell lineage, sex determination, and to 

 a lesser extent organ formation. 



In regard to techniques that have been used upon other invertebrates 

 and upon vertebrates it is of interest to note that (1) separation of early 

 blastomeres is of course impossible owing to the superficial cleavage, but 

 this aspect is covered by the foregoing techniques 2, 5, and 8; (2) vital 

 staining for marking egg regions has been tried unsuccessfully by Seidel, 

 and No. 2 above had to be used instead; and (3) tissue culture methods, 

 parabiotic twinning, and operative technique in the sense of trans- 

 plantations have not been successfully accomplished with insect eggs. 

 The tough chorion together with the turgidity and fluidity of the egg 

 contents hinders manipulation. 



NUCLEAR MOVEMENTS IN CLEAVAGE 

 AND BLASTODERM FORMATION 



Analysis of the motivating factor or factors in nuclear movements is 

 difficult because of the inability to separate cause from effect in observa- 

 tional data. The movement is not impeded b}^ repeated mitoses during 

 migration; in fact, Huettner (1935) says movement may in certain cases 

 be accelerated during mitosis in Drosophila. The movement of the 

 centrioles during mitosis shows that they cannot be the causative factors 

 even though they constantly lie toward the egg periphery in undividing 

 nuclei (Huettner, 1933; Strasburger, 1934). The repulsion tendency 

 cannot possibly function to cause centrifugal movement of the nuclear 

 sphere. 



Three possibilities remain: (1) active nuclear movement which pushes 

 the cytoplasm in front of it and draws that behind, (2) plasma streaming 

 which carries the nuclei along passively, and (3) some unknown attrac- 

 tive influence from the surface region. The only observation that can be 

 considered in favor of the first hypothesis is Strasburger's report that 

 plasma streaming does not occur in nonnucleated but seemingly uninjured 

 parts of the egg of CalUphora. But this is by no means conclusive 

 evidence even for this species, since he does not consider such questions 

 as the possible failure of local activation, altered viscosity, or other 

 possible invisible physicochemical changes in these cauterized eggs. 

 He attempts to circumvent the difficulty of ascribing active nuclear 

 movements to the nucleus by suggesting that the movement is due to the 

 protoplasmic island in whole or part, but this is unsupported and also 

 conflicts with his own data on the formation of the inner cortical layer 

 and with Seller's data to be discussed below. Another possibiUty that 



