EBIDRYOLOGY. 



centre of the radiations, a figure, which may be appropriately com- 

 pared (fig. 27) with a dumb-bell. It arises by the accumulation of a 

 large amount of homogeneous protoplasm around the poles of the 

 elongating nucleus, forming the two ends of the dumb-bell; the 

 poles may be regarded as if they were two centres of attraction. 

 The non-granular streak, representing the handle of the dumb-bell. 

 is the nucleus, which has meanwhile undergone a peculiar metamor- 

 phosis and has become indistinct. 



A more accurate knowledge of the nuclear metamorphosis may be 

 got by employing suitable reagents and dyes. By means of inter- 

 mediate stages, which may be disregarded here, there arisas out of 



Fig. 26. 



Fig. 27. 



Fig. 26. Egg of a Sea-urchin immediately after the conclusion of fertilisation, ft, Cleavage- 

 nucleus. 



I*if . 27. Egg of a Sea-urchin in preparation for division. The nucleus is no longer to be seen ; 



there hat arisen in its place a dumb-bell figure. 

 Both figures are drawn from the living object. 



the vesicular nucleus the nuclear spindle (fig. 31 B), which is a 

 typical structure for cell-division throughout the organic world. 

 Tli is (sp) consists of two substances, both of which, in my opinion, 

 are derived from the quiescent condition of the nucleus namely, 

 (1) of a non-diromatic substance, which does not show affinity for any 

 dyes, and (2) of the stainable nuclein or cliromatin. The non-chromatic 

 >u Instance forms extraordinarily fine, and therefore at times scarcely 

 niihle, "spindle-fibres" which are united into a bundle, and 

 ^m- i IM- to u spindle by the convergence of their ends to points. Tho 

 <>at in, on the contrary, has assumed the form of small individual 

 granules or chromosomes, which correspond in number with the 

 i If -fibres, and are so arranged that each granule adjoins a 

 He-fibre at it> middle point. In its totality, therefore, it con- 

 stitutes at the middle of the spindle a plate composed of individual 



