188 NUCLEOCYTOPLASMIC RELATIONS 



conditions of death are unusual. After irradiation of the females, 

 the eggs are oviposited, and meiosis proceeds at the usual rate. 

 Mitosis slowly takes place through two to four cleavages and 

 then nuclear division is blocked for several hours. At approxi- 

 mately the seventh or eighth hour after oviposition, feulgen-neg- 

 ative nuclei are seen here and there in the egg interior. When the 

 total number of nuclei reaches approximately 64 or 128 per egg 

 — the sixth or seventh cleavage — division is blocked completely. 

 The nuclei immediately begin to enlarge, occasionally reaching 

 a size 200 times their normal volume. The nuclei contain pro- 

 tein, but deoxyribonucleic acid, as shown by the feulgen reaction 

 and other methods, is lacking ( von Borstel, 1953a, 1955 ) . 



This answers the third question, which refers to critical periods 

 during the cleavage stages. When the egg is placed under stress 

 from radiation or breaking, some condition present at the sixth 

 cleavage appears to be some sort of barrier to further develop- 

 ment. Furthermore, it is of interest to note that, in those embryos 

 that get past this cleavage stage (for radiation effects, lower 

 doses are used), development will proceed to the blastula stage 

 at least. Critical periods during cleavage occur normally in Sciara 

 (Du Bois, 1933; Metz, 1938) where the "limited" chromosomes 

 are eliminated at the fifth or sixth cleavage and the sex chromo- 

 somes are eliminated at the seventh or eighth cleavage. That crit- 

 ical periods exist during rapid cleavage is apparent; the mecha- 

 nisms involved are unknown. It is clear from study of the broken 

 Habrohracon eggs that the organization of non-nuclear elements 

 is vital; it is apparent from the radiation experiment that the 

 nucleus is also involved. It seems possible that some basic interac- 

 tion between the nucleus and cytoplasm or some switch in 

 nuclear and cytoplasmic function occurs at this stage of develop- 

 ment in Habrohracon. 



The Nucleus and Cytoplasm in Determinate Development 



Depending on the insect, the age of the embryo, and the type 

 of experiment, early insect development has been shown to be 

 completely regulative, completely determined, or somewhere in 

 between. Seidel and his school (see Seidel et at, 1940, for review) 



