12 HUMAN ANATOMY. 



In anticipation of the consideration of the details of mitosis, it should be pointed 

 out that the process includes two distinct, but intimately associated and coinci- 

 dent series of phenomena, the one involving the chromatin, the other the centro- 

 somes and the linin. While as a matter of convenience these two sets of changes are 

 described separately, it must be understood that they take place simultaneously and 

 in coordination. The purpose of the changes affecting the chromatin is the accu- 

 rate and equal division of this substance by the longitudinal cleavage of the chroma- 

 tin segments ; the object of the activity of the centrosomes and the linin is to supply 

 the requisite energy and to produce the guiding lines by which the chromatin 

 segments are directed to the new nuclei, each daughter cell being insured in this 

 manner one-half of the maternal chromatin. 



The Prophases, or preparatory stages, include a series of changes which 

 involve the nuclear substances and the centrosomes and result in the formation of the 

 karvokinetic figure ; the latter consists of two parts, (i) the deeply staining chro- 

 matin filaments, and (2) the achromatic figure, which colors but slightly if at all. 

 The chromatin loses its reticular arrangement and, increasing in its staining affinities, 

 becomes transformed into a closely convoluted thread or threads, constituting the 

 " close skein ;" the filaments composing the latter soon shorten and thicken to form 

 the " loose skein." The skein, or spireme, may consist of a single continuous fila- 

 ment, or it may be formed of a number of separate threads. Sooner or later the 

 skein breaks up transversely into a number of segments or chromosomes, which ap- 

 pear as deeply staining curved or straight rods. A very important, as well as remark- 

 able, fact regarding the chromosomes is their numerical constancy, since it may be 

 regarded as established that every species of animal and plant possesses a fixed and 

 definite number of chromosomes which appear in its cells ; further, that in all the 

 higher forms the number is even, in man being probably twenty-four. During these 

 changes affecting the chromatin the nucleolus, or plasmosome, disappears, and, prob- 

 ably, takes no active part in the karyokinesis ; the nuclear membrane likewise fades 

 away during the prophases, the nuclear segments now lying unenclosed within the 

 cell, in which the cytoplasm and the nuclear matrix become continuous. 



Coincident with the foregoing changes, the centrosome, which by this time has 

 already divided into two, is closely associated with phenomena which include the ap- 

 pearance of a delicate radial striation within the cytoplasm around each centrosome, 

 thereby producing an arrangement which results in the formation of two stars or 

 asters. The centrosomes early show a disposition to separate towards opposite poles 

 of the cell, this migration resulting in a corresponding migration of the asters. In 

 consequence of these changes, the retreating centrosomes become the foci of two 

 systems of radial striation which meet and together form an achromatic figure known 

 as the amphiaster, which consists of the two asters and the intervening spindle. 

 Notwithstanding the observations which tend to question the universal importance 

 of the centrosome as the initiator of dynamic change within the cell, as held by Van 

 Beneden and Boveri, there seems to be litde doubt that the centrosome plays an 

 important role in establishing foci towards which the chromosomes of the new nuclei 

 become attracted. 



The nuclear spindle, which originates as part of, or secondarily from the 

 amphiaster, often occupies the periphery of the nucleus, whose limiting membrane by 

 this time has probably disappeared. The delicate threads of linin composing the 

 nuclear spindle lie within an area, the polar field, around which the chromosomes 

 become grouped. The chromosomes, which meanwhile have arisen by transverse 

 division of the chromatin threads composing the loose skein, appear often as 

 V-shaped segments, the closed ends of the loops being directed towards the polar 

 field which they encircle. Owing to this disposition, when seen from the broader 

 surface, the chromosomes constitute a ring-like group, sometimes described as the 

 mother lureath ; the same segments, when viewed in profile, appear as a radiating 

 group of fibrils known as the mother star ; the apparent differences, therefore, be- 

 tween these figures depend upon the point of view and not upon variations in the 

 arrangement of the fibres. 



The Metaphase includes the most important detail of karyokinesis, — namely, 

 the longitudiyial cleavage of the chromosomes, whereby the number of the latter is 



