10 



SUBSTAXCE COMPOSING THE CELL. 



Tlie fonnation of cells by segmentation may be traced witli comparative ease 

 in the ova of many invertebrata. The accomiianjang' figaire (fig. 6) represents 

 the several stages of the process in small species of the ascaris woiin. A, B. and 

 c are from the Axcarif; )iUirt>rc)iom, as observed by KoUiker. He found that, 

 after the genninal vesicle had disapiaeared, a new nucleus with nucleolus was 

 formed in its place : the segmentation then goes on as in the mammalian ovum 

 but the nuclei are visiljle from the fii'st. 



In many aninrals the segmentation process affects only a part of the contents 

 of the ovum. 



Fig. 0. —Division of the Yelk of Ascakis. 



A, B, (from Kolliker), ovum of Ascaris uigroveuosa ; d and e, that of Ascaris 

 acuminata (from Bagge). 



The Protoplasm of the Cell. — The substance of which the 

 embryonic cells, and all others which display similar vital contractility, 

 chiefly consist, is in reality clear and hyaline, but commonly contains 

 minute particles imbedded in the clear substance, which give it a 

 granular appearance (fig. 4, h). It is semi-fluid and viscid in con- 

 sistence, and in chemical constitution closely agi-ees with the albumi- 

 noid bodies, consisting, in fixct, principally of a substance allied to 

 myosin, the chief constituent of muscular tissue ; but in many animal 

 cells it doubtless also includes other organic principles, especially 

 fat, and glycogenous or amyloid matter. Proto])lasm is characterised 

 by properties which have been aptly termed " vital," since upon their 

 presence the life of the organism seems to depend. Chief among 

 these properties are those of assimilation and of irritability : indeed, 

 it is probable that the vital properties of the textures above enumerated 

 depend, in great measure or wholly, upon the protoplasm which they 

 contain. 



The Nucleus of the Cell.— The nucleus (fig. 4, c) is a round or 

 ovoid, clear, and apparently vesicular body, commonly situate near the 

 centre of the cell, and containing one or two strongly refracting 

 granules — the nucholi. — which are probably of a fatty nature. 



From the affinity which, in common with protoplasm, it possesses for certain 

 colouring matters, the nucleus has been supposed hy some eminent histologists 

 (and notably by Beale. who has applied the term •' germinal matter" to both) to 

 be identical in nature with that substance. Its behaviour, however, with many 

 reagents is altogether different ; and in general it may be said that it offers greater 

 resistanceto their action than the substance which surrounds it. The fact that 

 m the di\'ision of cells the segmentation of the nucleus appears to precede that of 

 the protoplasm, has been hold to be a strong argument in favour of the possession 

 by the nucleus of a considerable amount of vital activity, but it is impossible to 

 say whether this process may not be effected by means of the surrounding proto- 

 plasm. At all events there are. it is believed, no recent obsen'ations which would 

 tend to show the manifestation by the nucleus of any vital phenomena, unless 

 when associated with protoplasm, whereas the converse fact is well established. 



