D. J. SCOURFIELD ON MENDELISM AND MICROSCOPY. 409 



far as appearances go, merged into the general structure of the 

 nucleus. There is good indirect evidence, however, that fchey 

 nevertheless do retain their identity, for, apart from the fact that 

 their number remains constant, if differences in size, shape, 

 stainability, etc., occur, as they sometimes do, these peculiarities 

 are most faithfully carried on from cell-division to cell-division. 

 It is scarcely possible to imagine how this could be unless each 

 chromosome maintained its individuality. 



2. The existence of the chromosomes in pairs, one of each pair 

 being derived from one parent and the other from the other parent. 

 — The evidence for this depends upon the facts that when 

 chromosomes of different size, etc., exist, they occur in pairs in 

 each body-cell, that in the formation of the germ-cells only one of 

 each sort is allotted to each gamete, and that when fertilisation 

 takes place there is a pairing of the chromosomes with similar 

 peculiarities. 



3. The qualitative difference between the chromosomes contained 

 in each germ-cell. — The differences in size, etc., already mentioned 

 as occurring in some cases between the chromosomes, lend support 

 to this ; but until it is found possible to actually take out some of 

 the chromosomes from a fertilised ovum and watch the results,, 

 it will not be possible to get incontrovertible proof of the idea. 

 Boveri has carried out some ingenious experiments, however,, 

 which distinctly point to the necessity for the presence of at least 

 one complete set of chromosomes — i.e. a set such as occurs in each 

 germ-cell, equal to half the number in the body-cells — to ensure 

 normal development. By the double fertilisation of sea-urchins' 

 esrcrs he obtained three sets of chromosomes and two division- 

 spindles in one cell, which necessarily led to an abnormal partition 

 of the chromosomes in the daughter-nuclei. Following the 

 development of these eggs he found that it was of an abnormal 

 character. Altogether the conclusion seems justified that for the 

 production of a normal individual a definite group of chromosomes 

 is necessary, and that exactly this group exists in each germ-cell 

 and twice over in each other cell. 



•i. The separation of the jxiirs of chromosomes in the maturation 

 divisions so that each germ-cell contains one of each pair. — In spite 

 of the most conflicting opinions as to the details of the process, 

 and of considerable variations in the process itself, there now 

 seem to be good grounds for believing that the final result of the 



