THE SPEBMAT03ENhSI^ OP DOMESTfC MAMMALS. 233 



sijerrarttijgouia of the guinea-pig and ho concluded that this unmbor is probably 

 con"oct. 



Tlie existence of pairs of chromosomes : — Since the cliromosomes of tlui 

 mouse are of largo number and vary s) gi-adually in size, that it Ls diflicult 

 to determine them accurately by only cm"SJry oljsorvaticms, csi)ecially the 

 existence of the pairs among the chromosJines and the constant relation witli 

 regiu'd to their siz3 and form are the points which require a tlnjrough and 

 careful study. 



For the determinatiim of the cxLstence of the p.oirs among s imatic chromo- 

 sames, measurements liave lieeu made bj- some investigatore. Meves ('11) 

 in salamandei", Hajüce (,17) in the pig and Parmenter ('19) iu s^vae Amphilwa 

 (Amblystoma) measured the length of the chromosomes, while K-VINUKI ('19) 

 in silkworm measured the diameter and calculated the volume. 



In the mouse it is difficult to determiue the existence of the pairs of 

 chromosomes by the comparison of their length only, as they are not sti-aight 

 but of various shapes. Nevertheless, if the size of every clffomosome is 

 constant in any definite ^xiriod of the mitosis, and if it is possible to determine 

 the trne size of the chromosomes, it would be a very good method for tlio 

 dete-rmination of the presence of the pah's, Ijut it is very chfficult to detarmine 

 since the sha]^)es of the chromosomes vnrj so very much. 



As before stated, the writer therefore, tried to find out the jKiired 

 chromosomes, basing the dete>rmination on their shaj» and size, as he thought tliat 

 this would give more trustworthy results than either by culculating the length 

 or the breadth only (pp. 211). Three thhigs must, however, be considered 

 iu tliis method : these ai'e, 1. The difference in the degree of the condensation 

 of the particles composing the chromosomes, which may cause slight diiferencea 

 iu tbe relative size and form. 2. Tlie small difference iu size between 

 neighbouring chromosomes in the series of the chromosomes arranged according 

 to the size makes it sometimes impossible to demonstrate beyond all doubt the 

 presence of pairs among them. 3. It is possible tliat an error may occur 

 when the chromosomes are di'awn with the aid of a camera-lucida. 



In spite of these consideratioas, it will be seen from Table 11 where the 

 chromosomes are arranged according to their area, that the present method is a 

 most reliable one, as it affords on thi whole imdeniablc evidence that the chromo- 



