Ill DIFFERENTIATION 89 



germ-cells. Moreover, in fertilization the acrosome, sperm- 

 head or nucleus, and centrosome, alone are essential, for the 

 tail may be left outside. The functions of the acrosome 

 ensuring the entrance of the spermatozoon and of the 

 centrosome making the sperm-sphere are known : hence 

 the nucleus is the seat of the determinants of those characters 

 that can be transmitted by the male, and these are similar 

 to those transmissible by the nucleus of the female. At the 

 same time we know from the phenomena of merogony and 

 artificial parthenogenesis that both nuclei are not necessary, 

 but that one set of n chromosomes will suffice. 



We have now to consider the evidence for believing that 

 these n chromosomes are really different from one another. 



3. Roux long ago pointed out that karyokinesis looked like 

 an apparatus for simultaneously dividing and distributing 

 to two cells a number of qualitatively unlike bodies, but the 

 experimental proof of the dissimilarity has only recently 

 been brought forward by Boveri. 



This proof is based on the behaviour of dispermic eggs of 

 the Sea-urchin Strongylocentrotus lividus. 



The dispermy is caused, not by treatment with a poison 

 (as in the experiments of the brothers Hertwig), but merely 

 by adding a large quantity of sperm to the ova. 



Of these dispermic eggs the following types may be dis- 

 tinguished : 



I. The tetraster (each sperm produces a centrosome which 

 divides), followed by simultaneous quadripartition. There 

 are spindles between all four centres. 



a. Plane tetraster. The four cells lie in one plane, parallel 

 to the equator of the egg. 



Another meridional division gives eight cells in a ring. An 

 equatorial cleavage separates animal from vegetative blasto- 

 meres, and there follows the formation of sixteen mesomeres, 

 eight macromeres, and eight micromeres. 



b. Tetrahedral tetraster. 



There are never eight micromeres and macromeres, but 

 either six or four of each. 



II. The double spindle. 



