CLEAVAGE AXD DIFFERENTIATION 



87 



Fig. 37 



The equality of nuclear division during cleavage. A fertilised egg of Triton 

 taeniatus was constricted by a ligature, restricting the nucleus to the right-hand 

 half, in which cleavage has reached the 8-cell stage, while the left-hand half is 

 still undivided. A, At the i6-cell stage one of the cleavage-planes coincides with 

 that of the ligature, and 1/16 nucleus has passed across into the as yet undivided 

 left half. B, The ligature was then drawn tight so as to effect complete separation 

 between the two (lateral) halves. C, Each developed into a perfect embrvo (one 

 slightly further advanced than the other) ; 140 days after the operation, they were 

 identical. A nucleus of the i /16 stage is therefore equivalent to that of the whole 

 egg. (From Spemann, Zeitschr. Wiss. Zool. cxxxii, 1928.) 



