90 



DIFFERENTIATION 



III 



One sperm nucleus with its two centres and the spindle 

 between them remains apart from the other. The latter 

 conjugates with the female pronucleus. 



a. Both spindles lie in the same plane. 



b. They are at right angles to one another, that is tetra- 

 hedrally placed. 



These eggs usually divide into two, occasionally (Teichmann) 

 into four. On one side of the egg are nuclei with 2 n, on the 

 other nuclei with n chromosomes. (These double-spindle eggs 



a, o, c, J f 



FIG. 17. Diagram of one case of irregular chromosome distribution 



in a doubly fertilized egg ; lf j} c lt 



and 



2 , & 2 , c 2 , and rf> an( ^ 



c 3 , and c? 3 are the three complete, specific sets of unlike chromosomes. 

 (After Boveri, 1904.) 



have been referred to already in another connexion. To our 

 present purpose they are irrelevant.) 



III. The triaster. 



By shaking the eggs the division of one centre is prevented. 



a. Triaster proper, with spindles between all three centres. 



b. Amphiaster-monaster, in which one centre remains apart. 

 The triaster divides simultaneously into three by meridional 



divisions, and then into six by another meridional cleavage. 

 An equatorial division is followed by the production of twelve 

 mesomeres, six macromeres, and six micromeres. 



IV. Both centres may fail to divide. 



a. An amphiaster is formed between them. 



