200 



Embryogenesis: Preparatory Phases 



end of the egg while the part with the egg 

 nucleus remained undivided (as is to be 

 expected from the early experiments of 

 Ziegler and others showing that fragments 

 of fertilized eggs containing the egg nucleus 



and that cannot operate normally when the 

 pronuclei are separated by too great a dis- 

 tance within the egg. However, in the ab- 

 sence of demonstration of such contractile 

 processes many alternative interpretations 



0:10 (23°C) 



1:30 



3:00 



3:30 



4:00 



Fig. 67. A, Diagrams illustrating behavior of the pronuclei in physiological polyspermy in the salamander 

 Triton. Sequence is indicated by time (hours and minutes) after insemination. All four sperm that enter 

 develop' normally until 2:30, at which time the accessory asters attain their maximimi size, and one sperm 

 nucleus makes contact with the egg nucleus. At 3:00, when sperm and egg nuclei fuse, the accessory 

 sperm nucleus nearest the fusion nucleus shows signs of degeneration. At 3:30 to 4:00 all of the acces- 

 sory sperm nuclei progressively degenerate and the one in the animal hemisphere is pushed out of that 

 region by the cleavage amphiaster. (From Fankhauser, '48.) 



B, Diagrams illustrating constriction of a salamander egg by means of a hair loop. The unpigmented area 

 represents the location of the second polar spindle. The three darkly pigmented spots mark the entrance 

 points of the spermatozoa. In the constricted egg both fragments can develop into embryos, although nor- 

 mally the spermatozoon in the left fragment, upon failure to fuse with the egg nucleus, would degenerate. 

 (From Fankhauser, '34.) 



fail to divide while those with the sperm 

 nucleus do). Moore interprets the results of 

 these and similar experiments on the basis of 

 contractile processes that pull the separate 

 and the fused nuclei to their proper positions 



may be suggested. For example, the develop- 

 ing sperm aster could well be responsible for 

 failure of the sperm pronucleus to traverse 

 the narrow neck of the centrifuged egg. 

 Studies of physiological polyspermy in 



