700 A MANUAL OF VETERINARY PHYSIOLOGY 



important feature, without which the cell could not be reproduced. 

 The reproductive material lies in the nucleus, seen to consist of a 

 network of threads (Fig. 244, a), which, owing to their affinity for 

 staining material, are known as chromatin. Within the threads are 

 refractile bodies. In addition to the nucleus, the parent cell contains 

 another and much smaller cell, known as the centrosome, a body of the 

 utmost importance. The centrosome is surrounded by radiating 

 fibrils, which give it the appearance of a star (Fig. 244, a), and the area 

 surrounding it is spoken of as the attraction sphere. The reproduc- 

 tion of cells is brought about by indirect division, or mitosis, and the 

 change is initiated in the nucleus. The chromatin threads of which 

 this consists form larger threads by the process of fusion until the 

 network is lost, and there only remains a collection of bodies resem- 

 bling bent or curved rods (Fig. 244, b, c). These are the chromosomes, 

 and the number present in each cell of the body is always definite for 

 the particular species, but is not the same in all species of animals. 

 There are, for example, twenty-four chromosomes in the mature 

 somatic cells of man, the mouse, trout, salamander, and lily ; in 

 some of the threadworms there are two ; in some of the Crustacea 

 as many as 168.* The number of chromosomes in the horse is 

 twenty-six ;f in the donkey twenty-four; in the cow sixteen. No 

 matter what somatic cell is examined, the number is invariable 

 for any given animal. The importance attached to the number, 

 which is always even for every animal and plant sexually produced, 

 will shortly be evident. The chromosomes themselves are of extra- 

 ordinary interest. 



The first change preparatory to the division of any cell takes 

 place, as has been stated, in the nucleus, and while this is occurring 

 the centrosome has divided into two, one passing to opposite sides 

 of the nucleus (Fig. 244, b, c, d) ; these, with their star-like fibrils, 

 are known as astrospheres. The chromosomes being formed, the 

 cell wall of the nucleus disappears, and so liberates them (Fig. 244, d) ; 

 they are then arranged between the two poles formed by the astro- 

 spheres, placing themselves end to end in Indian file equatorially 

 (Fig. 244, e), and, having done this, they split longitudinally, and 

 each forms two (Fig. 244,/). Each split chromosome is attracted 

 towards the astrosphere nearest to it, so that an equal number of 

 chromosomes pass to each of these bodies. . The cytoplasm of the 

 cell has hitherto been an inactive witness of this remarkable 

 phenomenon. It now, however, divides into two portions (Fig. 244, 

 h, i, j), each engulfing an astrosphere with its attendant chromo- 

 somes, and in this way two cells are made out of one, which resemble 

 the parent in every respect, excepting that they only contain half 

 the amount of chromatin, though the full number of chromosomes 

 is present. The subsequent history of the new cell is simple. 

 The chromosomes come together, form a reticulum, surround them- 

 selves with a membrane constituting the cell wall of the nucleus, 

 and increase the amount of chromatin up to that originally existing 

 in the parent cell. 



Every animal originates from a cell which differs in no essential 

 particulars from the one just described. The process of multiplica- 

 tion by which cells form a lion or a mouse, an elephant or mole, a 



* ' Heredity and Disease,' by E. Le C. Lancaster, M.B., B.Ch., British 

 Medical Journal, February 5, 1910. 



f 'The Principles of Heredity applied to the Race-Horse,' by J. B. 

 Robertson, M.R.C.V.S., 1910. 



