APPENDIX B 



OX THE PART PLAYED BY THE NUCLEUS IN 

 DIFFERENTIATION 



(i) BOVERI has more recently (Zellen-Studien, vi, Jena, 1907) 

 published a very elaborate account of the irregularities produced 

 by dispermy in Echinoid eggs, in which are brought forward 

 still more facts in proof of the qualitative difference of the 

 chromosomes. 



As has been stated above, p. 263, dispermy is induced by 

 the simple expedient of adding a large quantity of sperm to the 

 eggs. The following types of dispermy are distinguished. 



A. Tetracentric, i. e. each sperm centre divides, 

 (i) Tetraster, with four spindles. 



(ii) Double spindle, i. e. the female and one male pronucleus 

 lie in one spindle, the other male lies aside in its spindle. 



B. Tricentric, one sperm centre remaining undivided, 

 (i) Triaster, a tripolar figure with three spindles. 



(ii) Monaster-amphiaster, the undivided sperm centre re- 

 maining apart with one sperm nucleus. 



C. Dicentric, neither sperm centre dividing. 



(i) Amphiaster, a spindle is formed between the two centres, 

 (ii) Double monaster : the centres remain apart, one with 

 one male, the other with the other male and the female 

 pronucleus. 



The segmentation of these eggs is as follows. 



The tetraster divides simultaneously into four, which may 

 either lie in one plane if the divisions are meridional, or be tetra- 

 hedrally arranged. In the first case another meridional division 

 ensues, followed by an equatorial, then eight micromeres are 

 formed, eight macromeres, and sixteen mesomeres. In the latter 

 case not more than three cells can share in the micromere region 

 and only four or six of these are produced. The triaster eggs, 

 having divided simultaneously into three (meridionally), sub- 

 sequently show six micromeres, six macromeres, and twelve 

 mesomeres. 



The segmentation of the double spindle eggs is interesting and 

 important. Usually the egg divides across the two spindles 



