NUCLEI OF ANIMAL AND VEGETABLE NUCLEI. 139 



(Plate XII, fig. 1). Both zone and nucleus increase in mass, 

 but the zones in the denser portions are smaller in relation 

 to the nuclei than in those which are less dense. The 

 protoplasm of the sphere enclosing the nucleus often presents 

 an undoubted radial arrangement (Plate XII, fig. 4). After 

 a time the nucleus becomes vacuolated, the hollows being 

 filled with fluid of a rosy tinge. The outer contour of 

 the sphere soon differentiates into a sldn (Hautschickt) , while 

 the remaining protoplasm of the sphere becomes reticular as 

 the cell grows (Plate XII, fig. 5). Not until neighbouring cells 

 have increased so as to allow of the direct contact of their 

 skins is it possible to demonstrate the presence of a sepa- 

 rating cellulose wall. After this stage has been reached and 

 the cells form a complete tissue, all further increase takes 

 place by cell-division. 



Of cell-division in the vegetable kingdom the case of 

 certain cells in the embryonal vesicle of Picea vulgaris may 

 be taken in illustration (Plate XII, figs. 7 — 12). On fertili- 

 sation, the original nucleus of the embryonal vesicle dis- 

 appears and four nuclei simultaneously present themselves 

 at the uj)per extremity (the lower in the diagrams) of the 

 vesicle. These are separated from one another and from 

 the rest of the vesicle by series or rows of granules (Plate 

 XII, fig. 7). 



Hereupon a process of true division begins. The border 

 of each nucleus becomes circular and ill-defined ; and in 

 the equatorial plane a disc or plate appears composed of a 

 single layer of upright, parallel, rod-like granules. On either 

 side of this nuclea?' disc [Kernplatte) the substance of the 

 nucleus is marked by strise Avhich converge towards the 

 poles of their respective halves (Plate XII, fig. 9), and 

 terminate there in a somewhat circular spot. 



The nuclear disc commences to thicken by the longitudinal 

 extension of its rods, the latter being drawn out into thin 

 fibres at their centres, while their ends travel in a direction 

 along the converging striae above mentioned, to meet together 

 at the circular polar spots. Here they form clear ellipsoid 

 structures of small size, which are in reality the nuclei of 

 the future segments (Plate XII, fig. 9). 



They do not long remain clear, but soon exhibit a striation 

 of granules in the direction of their shorter axis (Plate 

 XII, fig. 11). They surround themselves with zones of proto- 

 plasm and increase in size. 



Meanwhile the nuclear fibres (Kernfdden) present central 

 swellings which afterwards fuse into a second equatorial 

 disc or plate (Plate XII, fig. 10). This plate grows in area. 



