2 14 SOME PROBLEMS OF CELL-ORGANIZATION 



I. Xuclcus and Cytoplasm 



From the time of the earlier writings of Frommann ('65, '6f), 

 Arnold ('67), Heitzmann ('73), and Klein i^j?)), down to the present, 

 an increasing number of observers have held that the nuclear reticu- 

 lum is to be conceived as a modification of the same structural basis 

 as that which forms the cytoplasm. The latest researches indicate, 

 indeed, that true chromatin (nuclein) is confined to the nucleus.^ 

 But the whole weight of the evidence now goes to show that the 

 linin-network is of the same nature, both chemically and physically, 

 as the cyto-reticulum, and that the achromatic nuclear membrane is 

 formed as a condensation of the same substance. Many investi- 

 gators, among whom may be named Frommann, Leydig, Klein, Van 

 Beneden, and Reinke, have described the threads of both the intra- 

 and extra-nuclear network as terminating in the nuclear membrane ; 

 and the membrane itself is described by these and other observers as 

 being itself reticular in structure, and by some (Van Beneden) as 

 consisting of closely crowded microsomes arranged in a network. 

 The clearest evidence is, however, afforded by the origin of the 

 spindle-fibres in mitotic division ; for it is now well established that 

 these may be formed either inside or outside the nucleus, and 

 there is a pretty general agreement among cytologists, with the 

 important exception of Boveri, that both spindle-fibres and astral 

 rays arise by a direct rearrangement of the pre-existing structures.''^ 

 At the close of mitosis the central portion of the spindle appears 

 always to give rise to a portion of the cytoplasm lying between the 

 daughter-nuclei; and in the division of the (tg2, in the sea-urchin 

 I have obtained strong evidence that the spindle-fibres are directly 

 resolved into a portion of the general reticulum. These fibres are 

 in this case formed inside the nucleus from the linin-network ; and 

 we have therefore proof positive of a direct genetic continuity be- 

 tween the latter and the cytoplasmic structures. But more than this, 

 I have found reason to conclude that in this case a considerable 

 part of the linin-network is derived from the chromatin, that the 

 entire nuclear reticulum is a continuous structure, and that it is no 

 more than a specially differentiated area of the general cell-network 

 {'95, 2). This conclusion finds, I believe, a very strong support in 

 the studies of Van Beneden, Heidenhain, and Reinke reviewed 

 beyond (p. 223) ; but the bearing of these only becomes plain after 

 considering the morphological differentiations of the nuclear net- 

 work and its transformations during mitosis. 



' Cf. Hammarsteii ("95). 



-The long-standing dispute as to tlie origin of the nuclear menihranc (whether nuclear 

 or cytoplasmic) is therefore of little moment. 



