MATUKATION OF UVUJ\l IN ECHINUS ESCULENTUS. 185 



irregular or feathery structure, while the uucleolus generally 

 is less deejily stained and vacuolated (fig. 2). Finally, when 

 the nucleus is fully grown and maturation imminent, we find 

 the contrast is exactly the opposite of that described for the 

 young nucleus. The network is intensely black, consisting 

 of particles of chromatin arranged in a very intricate and 

 irregular fashion, while the nucleolus parts with the stain 

 very readily, and is left as an almost colourless, apparently 

 empty vesicle. Soon after the resolution of the nuclear 

 membrane it disappears from view. Very similar changes 

 are described in many other forms, — for instance, in the 

 Turbellariaus, according to Francotte (1897) ; in Polychcerus, 

 according to Gardiner (1898) ; and according to Gathy (1900), 

 in Tubifex (an Annelid) the nucleolus loses its capacity for 

 staining with iron hgematoxyliu at the end of the growth period. 

 It is difficult to resist the conclusion that the chromatin 

 substance is at first confined to the nucleolus, and later leaves 

 it to form the chromatic basis of the nuclear network us a 

 whole, and therefore also of the future chromosomes. The 

 fate of the nucleolus in Echinoderm eggs has been variously 

 interpreted. Derbes (in 1847) thought it was directly 

 converted into the pronucleus of the mature egg, and 

 Hertwig (1877) took the same view. Fol (1877) and 

 Flemming (1882), however, proved that the chromatin 

 itself became the future nucleus, after it was provided 

 with a new nuclear membrane. Recently Caruoy and 

 Le Brun (1899) have maintained the view that in the 

 amphibian egg where there is no chief nucleolus, but a large 

 number of smaller ones, certain of these become converted 

 into the future chromosomes, thus reverting to the older 

 view of Schultze (1887). It seems certainly true, as said 

 above, that the chemical substance which is lodged in the 

 nucleolus in the early ovum becomes later distributed into 

 the germinal vesicle, and so indirectly goes to form the 

 chromosomes. Hartmann (1902), for Asterias glacialis, 

 describes the chromosomes passing directly out of the uucleo- 

 lus, the remainder of the nuclear reticulum being rejected. 



