114 COMPARATIVE ANATOMY 



of equal masses, called chromosomes, and it is a striking and 

 important fact that every species of plant or animal has a 

 fixed number of chromosomes, which number recurs in every 

 division of every tissue-cell in the body. The number may be 

 very large — it is said to be one hundred and sixty-eight in a 

 certain crustacean known as Artemia salina, — or it may be very 

 small, as in the thread-worm parasitic in the intestines of the 

 horse {Ascaris megalocephald), but in all animals which are 

 reproduced sexually it is an even number. 



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 Hes wholly outside the nucleus, and 

 is formed entirely under the influence of the centrosomata. 



The spindle and the two asters surrounding the centro- 

 somata do not easily stain with the ordinary dyes, and hence 

 are often called the achromatic figure, or amphiaster, but the 

 centrosomata 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 



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