216 THOMAS H„ ERYCE. 



olliers have attributed to it. If we look upon the tetcad as 

 a single chromosome longitudinally divided, then we cannot 

 get beyond the statement that during maturation the chro- 

 matin substance, which is retained to form the chromosomes, 

 condenses into masses, whicli are half the number of the 

 segments characteristic of the cleavage nuclei, and that these 

 masses adopt a special form in the prophases of the first 

 division, preparatory to the occurrence of a double longi- 

 tudinal splitting. 



It would be tedious and unprofitable to attempt a complete 

 recapitulation of all the cases described. In many forms, 

 owing to the difficulty of obtaining an absolutely complete 

 series of stages, the evidence is incomplete, while in others 

 the minuteness of the chromosomes is a barrier to finer 

 analysis. I must, however, refer briefly to the facts as repre- 

 sented by the botanists regarding the heterotypical division. 



In the earlier days of investigation into the mitosis occur- 

 ring in the pollen mother-cells of highei- plants, Strasburger 

 (1888) and Guignard (1891) described a longitudinal splitting 

 at the beginning of each division, and in regard to reduction 

 of the chromosomes they did not find that the pollen mitoses 

 differed from the process in vegetative cells. 



Belajeff (1894) was the first to point out that the V-shapcd 

 figures of the heterotype were not due to the rods or rings 

 being curved progressively in their ascent to the poles of the 

 spindle. 



Farmer (1895), as I have already said, described an apparent 

 double longitudinal cleavagesimultaneouslyprogressing,bnt in 

 his paper in conjunction with Moore he elaborated the idea that 

 the double rod of the prophase was produced by bending of 

 the ring on itself and the fusion of the two halves. In the 

 metaphase the rods were separated along the plane of fusion, 

 so that only a single longitudinal cleavage was involved, and 

 the separating elements were the original daughter chromo- 

 somes. He held that there was a longitudinal splitting of 

 the chromosomes in the second division. 



Strasburger (1895) gave an explanation involving two 



