No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 523 



nuclear membrane. Schwalbe ('76) found in the nuclei of 

 various vertebrate embryos that when the nucleoli first arise 

 they appear as thickenings of the inner surface of the nuclear 

 membrane. 



From these observations I conclude, accordingly, that the 

 nucleolar substance, in many if not all cells, has an extranu- 

 clear origin ; and that, though it may undergo a chemical change 

 after entering the nucleus, it can be regarded neither as a secre- 

 tion nor as an excretion of the latter. In making this con- 

 clusion I can corroborate the views of only one investigator, 

 namely, Korschelt ('89), though he changed this opinion in a 

 later paper ('97). He concluded that the nucleolar substance 

 stands in some connection with the nutritive processes of 

 the cell, and that the nucleus probably derives it from the 

 cytoplasm. 



Other views on the origin of the nucleolus (those of Hacker 

 have already been mentioned) : Auerbach ('74a, '76) first supposed 

 the nucleolus to be cytoplasmic in origin ; more recently ('90) 

 he appears to champion its nuclear origin. Rhumbler ('93) 

 assumes that the " Binnenkorper " of Protozoa are products of 

 the nucleus, but he does not attempt to decide whether those 

 of the Metazoa have a similar origin. Strasburger ('82b) also 

 postulates a nuclear origin for the nucleolus, and assumes that 

 its substance is allied to chromatin. Jordan ('93) holds that 

 the nucleoli probably arise from the chromatin threads. Flem- 

 ming ('82) considers them to be *' specifische Produkte des 

 Kernstoffwechsels." Schwalbe ('76) supposes the nucleolar 

 substance to be at first identical with that of the nuclear mem- 

 brane, since he found it to arise as thickenings of the latter. 

 C. Schneider ('91) supposes it to be a metamorphosed portion 

 of the chromatin. Leydig ('83) concludes that the nucleoli are 

 portions of the chromatin reticulum. Guignard ('85) assumes 

 that they are derivatives of the chromatin filaments. Watase 

 ('94) considers them to be metabolic products of the cell, but 

 he gives no detailed observations in regard to their mode of 

 formation. Mertens ('93) and Retzius ('81) consider them to 

 arise by concentration of the chromatin reticulum. 



