410 1). J- SCOURFIELD ON MENDELISM AND MICROSCOPY. 



maturation divisions and concomitant phenomena is to give to 

 each germ-cell one chromosome of each pair normally present in 

 the body-cells. In outline the process, or rather one type of the 

 process, is as follows. The cells destined to produce the germ-cells 

 contain, like all the other cells of the body, the number of 

 chromosomes, 4, 8, 12, 16, 24, 32, or whatever it may be, 

 normal to the species. Immediately preceding the first of the 

 two cell -divisions directly leading to the production of ripe germ- 

 cells, known as the maturation divisions, the chromosomes are 

 found to be split longitudinally. Now follows a kind of conjuga- 

 tion of the chromosomes belonging to each pair, so that bodies 

 consisting of four closely approximated elements, and known as 

 " tetrads," are seen in the place of double the number of original 

 chromosomes. Each tetrad, therefore, pretty certainly contains 

 two elements derived from the male and two from the female 

 parent. In the cell-division which now takes place the tetrads 

 are divided into two " dyads," one being allotted to each daughter- 

 nucleus. Another cell-division follows quickly, and each of the 

 two constituents of each dyad is moved into a separate nucleus. 

 Four distinct germ-cells have thus been formed from every original 

 mother-cell,* and the chromosomes have been treated in such a 

 way that half the germ- cells contain chromosomes derived from 

 one parent and half of them chromosomes from the other parent. 

 As there is no reason for thinking that all the chromosomes 

 from each parent are systematically passed on together into the 

 germ-cells — all the evidence, in fact, being against this idea — it is 

 reasonable to suppose that it is a matter of chance whether any 

 particular germ-cell contains all or none or any intermediate 

 proportion of the chromosomes of any one parent. 



It will be apparent from the foregoing remarks that, if the 

 very moderate assumption be made that the factors determining 

 any pair of contrasted characters, such as the round and wrinkled 

 shape of peas, etc., are situated in corresponding chromosomes 

 in the germ-cells of the two parents, then the phenomena of 

 hybridisation as worked out by Mendel can be easily translated 

 into terms of cytological processes. So far so good. But there 

 remains this difficulty, that the number of chromosomes charac- 



The only difference in the formation of the male and female gametes 

 is that in the former all four (Lmghter-cells become functional, while in the 

 latter only one of the four becomes an ovum. 



