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 (’ 74 a, ’ 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 (’ 82 b) 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. 
