GENERAL INTRODUCTION. 



the interspaces of which are filled with a mere liquid substance, the cell-sap or cytolymph. Along the fibres of the reticulum, 

 and especially at its nodal points, are found minute rounded bodies known as microsomes. The nucleus is a rounded 

 vesicle, surrounded by a distinct membrane and traversed by a reticulum composed of linin fibres, which are probably 

 continuous with those of the cyto-reticulum, and of the same general nature. Suspended in the linin network is a 

 coarser network composed of deeply staining chromatin, which is, in many cases, in the form of distinct granules 

 embedded in the linin network. A deeply staining rounded body, the nucleolus, is often also present; but this appears to 



be inconstant. 



In many cases, but probably not in all, a portion of the cyto-reticulum is condensed into a definite rounded body 

 lying beside the nucleus and known as the centrosome or ceutrosphere (Text-fig. V. A). Its interior sometimes contains 

 one or more minute rounded bodies, known as centrioles, but in other cases (as apparently in Toxopnetisles), there is no 

 centriole, the interior of the sphere consisting merely of a granular or reticular mass. 



Karyokinesis. — We may now consider briefly the apparatus, by means of which the fertilized egg divides. The usual 



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B 



Fig. IV. 



l-ic IV — \ The two aerm-iiuclei in the egg of the gastropod Plerotrachea. Highly magnified, after Boveri. E, the egg-nucleus; S, the sperm-nucleus, each containing sixteen 



elongated chromosomes. PB, the polar bodies. The centrosome has divided into two to form an amphiaster. Its origm m this animal has not yet been determmed. 

 i;. Later stage, showing the fully developed amphiaster. Above it lie the sixteen maternal chromosomes, below it the sixteen paternal, the nuclear membranes having disappeared. 



form of cell-division (from which the cleavage of the ovum differs only in the fact that it is preceded by a fusion of two 

 cells) involves a complicated process known as karyokinesis or mitosis. This is characterized by the appearance of a 

 structure known as the karyokiuctic figure (Text-fig. V. B), derived partly from the nucleus and partly from the surrounding 

 cell-protoplasm or cytoplasm. The karyokinetic figure consists of two elements. One of these, the achromatic figure or 

 amphiaster, consists of a fibrous spindle-shaped structure or spindle, at either pole of which is a star or aster consisting 

 of fibres or rays radiating into the cytoplasm, the whole figure strongly suggesting the arrangement of iron filings about 

 the poles of a horseshoe magnet. The centre of each aster is occupied by a rounded mass known as the centrosome or 

 centrosphere, which is derived from that of the resting-cell, and like the latter may contain one or more centnoles. 

 The entire substance of the amphiaster is often designated as archoplasm, but this term is strictly applied to the 

 substance of the astral rays and spindle-fibres alone. The second, or chromatic, portion of the karyokinetic 

 figure is derived from the nucleus, and consists cf chromosomes like those arising from the germ-nuclei, which are 

 grouped about the equator of the spindle. As the cell prepares for division each chromosome splits lengthwise into two 

 haUes, which diverge to opposite poles of the spindle, and here each group of daughter-chromosomes finally gives rise to 



