R. C. VON BORSTEL 181 



approximately 1024 nuclei are present, the first signs of blastula- 

 tion appear. Some cleavage nuclei remain in the egg interior 

 while the great majority migrate to the periphery. The nuclei in 

 the interior stain more intensely and, as cells, are known as yolk 

 phagocytes or vitellophags (see Sonnenblick, 1950, for review). 

 The nuclei at the periphery undergo several more synchronous 

 divisions, cell membrane formation takes place, and tissue dif- 

 ferentiation begins. 



Several questions can be asked about relations between the 

 nucleus and cytoplasm. Our work with Habrohracon and studies 

 done in other laboratories on other insects have provided answers 

 for some of these questions. 



1. The four meiotic nuclei are apparently all genetically 

 equipotent. Since all four remain inside the egg, why does only 

 one take part in further development? 



2. Is there any condition by which accessory sperm can take 

 an active part in development? 



3. Can the young embryonic system be regarded as a pool in 

 which mitosis freely occurs (the "pure culture of mitoses" of 

 Zeuthen, 1951) until blastulation, or do critical periods exist dur- 

 ing the cleavage stages? 



4. What component of the cytoplasm is the agent of deter- 

 minate development? 



5. Is there any general method by which one can distinguish 

 between action of the nucleus and the cytoplasm in bringing 

 about an embryonic event? 



Genetic Control of the Meiotic Block 



Genetic Mosaics. During a routine testing following an ir- 

 radiation experiment, a curious mutant was discovered. When 

 this mutant stock was inbred, a relatively high incidence of mo- 

 saicism resulted. These are mosaics in the genetic rather than the 

 embryological sense. In this stock, a virgin female heterozygous 

 for a semidominant body color gene (designated as "lemon 

 lethal"), which is a pupal lethal when homozygous, will occa- 

 sionally produce haploid male offspring that are made up of 

 nuclei carrying the normal allele and nuclei carrying the mutant 



