R. C. VON BORSTEL 



187 



dehydration, 47 of 63 (74.6%) of the eggs were capable of 

 further development. Thirty-three of 47 (70.2%) of the eggs 

 examined proceeded as far as the sixth or seventh cleavage stage. 

 Nineteen of 36 (52.8% ) of the eggs reach the blastula stage. Of 

 those that develop at all but still die before blastulation, most die 



Table I. Development in Habrobracon Eggs Broken at Completion of Meiosis 

 (30 minutes after oviposition at 30°) 



between the fourth and eighth cleavage stages. It would appear 

 from these limited data that the stages around the sixth cleavage 

 are partially critical for nuclei in rapid division. 



Another characteristic of the sixth or seventh cleavage stage 

 that indicates a critical period of embryogeny is illustrated by 

 a specific action of radiation or nitrogen mustard on eggs or 

 sperm manifested at that time (von Borstel, 1952, 1953b, 1955). 

 When metaphase I eggs are irradiated with 10,000 r, a dose 

 below that which is injurious to cytoplasm as indicated by in- 

 duced androgenesis (A. R. Whiting, 1955), nearly all the em- 

 bryos die at the same stage, the sixth or seventh cleavage. The 



