46 THE PHYSICAL BASIS OF INHERITANCE 



pig, and in man the number is said to be 48, and the same 

 number is found in some snails. In the grasshopper it is 12 ; 

 in the hepatic Pallavicinia and in some of the nematodes, 8 ; and 

 in A scans, another thread-worm, 4 or 2. In the crustacean 

 Artemia it is 168. Under certain circumstances, it is true, the 

 number of chromosomes may be less than the normal in a given 

 species ; but these variations are only apparent exceptions 

 [p. 87, Wilson]. The even number of chromosomes is a most 

 interesting fact, which, as will appear hereafter [p. 205, Wilson], 

 is due to the derivation of one-half the number from each of the 

 parents." 



2. About 1883, Van Beneden made the important discovery 

 that the nuclei of the ovum and of the spermatozoon which 

 unite in fertilisation contain each one-half of the number of 

 chromosomes characteristic of the body-cells. This has been 

 confirmed in regard to so many plants and animals that it may 

 now be regarded as a general fact. The student should refer 

 to the' partial list given by Wilson (1900, pp. 206-7), where 

 it will be seen that if the somatic nuclei have 12, 16, 18, or 24 

 chromosomes, the germ-nuclei have 6, 8, 9, or 12 respectively. 

 A striking case is found in the large thread- worm (Ascaris megalo- 

 cephala) of the* horse, which occurs in two varieties, the one, 

 var. univalens, with two chromosomes in its body-cells has one 

 chromosome in its germ-nuclei ; the other, var. bivalens, with 

 four chromosomes in its body-cells, has two chromosomes in its 

 germ-nuclei. 



3. If each of the nuclei which unite in fertilisation has only 

 half as many chromosomes as are characteristic of the species, 

 it follows that a reduction of the number must take place in the 

 history of the germ-cells, and this is the outstanding fact in 

 the process of maturation. Alike in the history of the egg 

 (oogenesis) and in the history of the sperm (spermatogenesis), 



