ii 4 COMPARATIVE ANATOMY 



ful whether the number of chromosomes in tissue-cells is as 

 constant as was at one time supposed. * 



At the time when the chromosomes are formed the nuclear 

 membrane has generally been absorbed, and has disappeared, 

 leaving the chromosomes apparently free in the cytoplasm, 

 and at the same time an important and remarkable structure 

 makes its appearance in the position formerly occupied by the 

 now broken-up nucleus. This is the spindle, composed of a 

 number of fibrillar threads converging at each end of the 

 spindle towards the centrosomata, and diverging from one 

 another in the centre, or, as we may call it, the equator of the 

 spindle. There is some doubt as to whether the spindle is 

 formed out of the substance of the cytoplasm or out of the 

 achromatic substance of the nucleus. In a large number of 

 cases, at all events, the spindle fibres appear to be formed in 

 two groups which have different origins : an internal group, 

 forming the axis of the spindle, is formed from the linin threads 

 of the nucleus, and an external group, forming the periphery 

 of the spindle, is derived from the cytoplasm, and probably 

 from that modified part of it which has been described as the 

 attraction sphere or centrosphere. It should be noticed that 

 in some cases e.g. the tissue-cells of the salamander the 

 spindle at its first origin lies wholly outside the nucleus, and 

 is formed entirely under the influence of the centrosomata. 



The spindle and the two asters surrounding the centrosomata 

 do not stain easily with the ordinary dyes, and hence are often 

 called the achromatic figure, or amphiaster, but the centro- 

 somata themselves stain readily and intensely with certain dyes. 



However the spindle may be formed, its eventual relation to 

 the chromosomata is the same. The latter become attached 

 to the spindle fibres in such a manner as to form an equatorial 

 ring round the spindle. The whole of the stages which have 

 been described Constitute what is known as the prophase of 

 mitosis, and may be regarded as preparatory to the next and 

 most essential step, the metaphase. This consists in the divi- 

 sion of the chromosomata into equal halves. (Fig. 23, .) If 

 they are rod-shaped or horseshoe-shaped the chromosomata 

 split into two lengthwise, if round or ovoid they simply divide 



* J. B. Farmer and D. Shove, On the structure and development of the 

 somatic and heterotype chromosomes of Tradescantia virginica. Quart. 

 Journal of Microscopical Science, xlviii. (1905) p. 562. 



