Numerical Reduction of Chromosomes 237 



is so constant that one feels almost inclined to call the pro- 

 nuclei prothallial nuclei. 



At the moment when the two prounclei separate, single 

 nuclei appear in place of the double nuclei, and the double 

 number of nuclear threads is thereby reduced to a single 

 one. This process is usually called the numerical reduc- 

 tion of the chromosomes ; but this imposing name means 

 nothing but the separation of two nuclei which had so far 

 worked together for a period. It is like the parting of 

 two persons who have walked along together for a while, 

 and will be looking for other companionship presently. 

 And this they achieve by fertilization. 



This parting has been minutely studied by numerous 

 investigators. It has the appearance of a nuclear division 

 of a very special nature, and is frequently called the reduc- 

 tion-division, or heterotypic nuclear division. It is neces- 

 sarily accompanied by a cell-division, since the two sepa- 

 rated pronuclei can only part in separate cells, but this 

 cell-division does not always follow immediately, but 

 only after a second essentially normal division of the 

 nuclei. There result, in that case, four sister-cells instead 

 of the usual two. 



Shortly before their separation, the chromosomes lie 

 together in pairs, always one in the paternal pronucleus 

 united with the corresponding thread of the maternal 

 pronucleus. They are placed lengthwise side by side. 

 Hence the separation evidently occurs by a longitudinal 

 line, and, in by far the greatest number of cases, this so- 

 called longitudinal splitting of the chromosome-pairs has 

 been observed in the origination of the prouclei. It is 

 true that this does not always succeed at a first glance, 

 and it is right here that the differences of opinion between 

 different investigators have blurred the picture for a long 



