SECT. 8.] PRIMORDIAL UTRICLE OF PLANTS. I 5 



cumscribed forms, and their resemblance to elementary vesicles, 

 as well as the circumstance that in certain cells, particularly in 

 ova and ganglionic globules, a larger or smaller cavity, tilled 

 with a clear fluid, is developed in them. The chemical compo- 

 sition of nucleoli is unknown. Their external aspect, their resem- 

 blance to elementary vesicles, their disappearance in caustic alka- 

 lies, and their insolubility in acetic acid, would imply that 

 they are composed of fat; the membranes, as in the element- 

 ary vesicles, may be a proteine compound. Nucleoli occur in 

 the great majority of nuclei, so long as the latter are young, 

 and, in many, as long as they exist; yet there occur, although 

 rarely, nuclei in which nucleoli cannot be recognised definitely, 

 or become distinct only at subsequent periods ; and accordingly, 

 the nucleolus cannot, for the present, be so unconditionally re- 

 garded as an essential constituent of the cell as the nucleus. 

 Usually a nucleus contains but one nucleolus ; frequently there 

 arc two, seldom three, and in very isolated cases four or five are 

 present, which then either lie eccentrically or free in the nucleus. 



Quite recently, Donders, in a very remarkable paper (see infra), has 

 asserted, that all cell-membranes consist of one and the same, or at least of 

 very closely allied, substances, which agree in their properties with the sub- 

 stance of elastic tissue. I, for my part, am of opinion, that all animal cell- 

 membranes consist originally of the same material, and, indeed, of a proteine 

 compound; but that, in consequence of subsequent metamorphoses, differ- 

 ences of composition and reaction may arise. Thus many membranes become, 

 in the course of time, more resistant, and approximate, as Donders correctly 

 states, to elastic tissue ; others become transformed into gelatiniferous tissues, 

 like those of the formative cells of connective tissue ; others, again, into synto- 

 nine, as in the smooth muscular fibres, others into horn, etc., etc. If we adopt 

 a proteine compound as the primitive cell-membrane, as we are constrained to 

 do from the reaction of young cells and embryonic parenchymas, there 

 results an agreement with vegetable cells ; seeing that in this case the pri- 

 mordial utricle of the latter, which consists of a proteine substance, may be 

 regarded as the analogue of the animal cell-membranes ; whilst the cellulose 

 membrane appears as a secondary formation, as a product of excretion. This 

 may be the case in the animal tissues of the Tunicata, which are composed 

 of cellulose ; in which case, my assertion, that, in the latter, cell-membranes 

 composed of cellulose occur — and that of Schacht ( Midi. Arch. 1 8 5 1 ), that 

 they are nitrogenous — would be compatible. If the future justifies this 

 comparison of animal-cells with the primordial utricle of plants, which I 

 do not doubt, all the chemical metamorphoses of the cell-membranes would, 

 very probably, be owing to deposits which are precipitated on their outer side, 

 similar to the cellulose in plants ; so that, besides the original proteine 

 membrane, other secondary elastic membranes, or gelatinous envelopes, 

 etc., would require to be distinguished. In this case we could say, that even 



