52 



EMBRYOLOGY. 



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 

 largo 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 arises out of 





Fig. 26. 



Fig. 27. 



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

 nucleus. 



Fig. 27. Eger of a Sea-urchin in preparation for division. The nucleus is no longer to be seen ; 



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

 Both figures are drawn from the living object. 



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

 typical structure for cell-division throughout the organic world. 

 This (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-chromatic substance, which does not show affinity for any 

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

 substance forms extraordinarily fine, and therefore at times scarcely 

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

 give rise to a spindle by the convergence of their ends to points. The 

 chromatin, on the contrary, has assumed the form of small individual 

 granules or chromosomes, which correspond in number with the 

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

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

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



