KARYOKINESIS. 191 



DOUBLE SKEIN OR DISPIEEM - { J Opea skein in daughter nuclei. 



( 9. Close skein in daughter nuclei. 

 NETWOEK OK RETICULUM ... 10. Resting condition of daughter nuclei. 



Historical. 1 The existence of utricles or saccules enclosed by a membrane was recognized 

 in the tissues of plants as long ago as the latter half of the 17th century (by Hooke, 

 Malpighi, Grew, and Leeuwenhoek), and a nucleus was noticed and described by Fontana 

 about a century later. At the beginning of the present century, the cellular constitution of 

 plants was further studied and described by Mirbel and Turpin ; but it was not until the 

 third decade of the century that the improvements which had taken place in the microscope 

 led to the general recognition of the fact that, amongst plants at least, the higher organisms are 

 entirely composed of cells, each of which is essentially formed of a membrane enclosing cell- 

 contents and contains a nucleus (R. Brown, Schleiden, 1831 1838). This generalization was 

 extended to the animal tissues by Schwann, in a remarkable work published in German in 

 1839 (Microscopical Researches into the Accordance in the Structure and Growth of Animals 

 and Plants. Sydenham Society's Translation, 18-47), in which he further showed that in all 

 probability every cell is derived from a pre-existing cell 



The researches of Schwann were so widely extended and the evidence he adduced was so 

 conclusive that his ideas, under the name of the " cell-theory " still remain as the accepted 

 doctrine of the constitution of plant and animal organisms. The term protoplasm was applied 

 by Purkinje to the substance of animal cells in 1840, but first came into extensive use after 

 its employment by v. Mohl, in 1846, who applied it to the living substance of the plant-cell. 

 The material itself, with all its most prominent characteristics as displayed in Infusoria, was 

 however described, in 1835, under the name of " sarcode," by Dujardin, the accuracy of whose 

 description has, it will be seen, left but little for subsequent observers to add : " I propose 

 to name sarcode that which other observers have termed a living jelly, a substance glutinous, 

 diaphanous, homogeneous, refracting light a little more than water, but much less than oil, 

 extensible and ropy like mucus, elastic and contractile, susceptible of spontaneously forming 

 within itself spherical cavities or vacuoles which become occupied by the surrounding liquid 



Sarcode is insoluble in water, but is eventually decomposed by it, leaving a 



granular residuum. Potash does not dissolve it suddenly like mucus or albumen, and seems 

 simply to hasten its decomposition by water ; nitric acid and alcohol immediately coagulate it 



and render it white and opaque The most simple animals, such as amoeba3 and 



monads, are entirely composed, at least to all appearance, of this living jelly. In higher 

 infusoria it is enclosed in a loose integument which looks like a network on its surface 



Sarcode is found in ova, zoophytes, worms, and in other animals ; but it is here 



capable of assuming with age a degree of organization more complex than in animals at the 



bottom of the scale Sarcode is without visible organs and has no appearance of 



cellularity ; but it is nevertheless organized, for it emits various prolongations along which 

 granules pass and which are alternately extended and retracted : in one word, it possesses 

 < life.' " 



Gradually, both in plants as well as in the lower animals, it came to be generally recog- 

 nized that the sarcode of Dujardin and the protoplasm of v. Mohl are endowed with similar 

 attributes, and a cell was defined as composed of structureless protoplasm, endowed with 

 irritability and contractility, containing a nucleus and enclosed by a cell-membrane. That 

 a cell-membrane is, however, not an essential character and is often absent, especially in 

 animal cells, was shown by Leydig (in 1856), and was especially emphasized by M. Schultze 

 ,and by Briicke (in 1861). Even at the present time the definition of a cell proposed by 

 Leydig still holds good " a mass of protoplasm furnished with a nucleus." 



Up to 1865 protoplasm was universally held to be homogeneous and structureless. 

 Attention was drawn by Frommann, however, to a fibrillar structure in the protoplasm of 

 many cells, and such structure was regarded by him as of universal occurrence. This view 

 was somewhat later expanded by Heitzmann. Klein, and others, who described a reticular 

 structure as occurring in all protoplasm, but it was by no means certain that the structure 

 described might not have been produced by the reagents which were employed to exhibit it ; 

 for it must be borne in mind that precisely such a reticulum as .is exhibited in protoplasm 

 which has been treated with alcohol or chromic acid can be equally well produced in solutions 

 containing albumen or mucus. Nevertheless, it is the opinion of many histologists at 

 the present day who have given special attention to the subject (Leydig, Kupffer. Flcmming, 

 Carnoy) that protoplasm invariably contains a reticulum, although others, and probably 

 a smaller number, still regard the reticular or spongy structure as non-essential to its 

 constitution (Kollmann, Strasburger, Schwartz). The reticular structure of the nucleus, 

 although it had been previously described by various observers, was for the most part regarded 

 as merely a localized and specialized part of the general cell-reticulum. The existence and 



1 The following account is mainly derived from the "Histoire de la Cellule," given by Carnoy 

 (Biologic Cellulaire, Fascicule 1, 1884). 



