MOTIONS OF THE PROTOPLASM. 



xix 



walls of the capillary vessels have been observed to multiply in this way in 

 the formation of new (morbid) growths, as is shown in fig. xiu. 



Fig. XIII. CAPILLARY VESSEL FROM A FIBROID TUMOUR ; MAG- Fig. XIII. 



NIFIED (adapted from C. 0. Weber, Virch. Arch. 1864). 



a, a, nuclei on the wall of the capillary ; 6, nucleus after 

 division into two ; c, group of nuclei surrounded by protoplasm ; 

 d, new cells ; e, cells of connective tissue. 



Seeing the successive generations of cells which proceed from 

 a single one in the ovum, and the propagation of cells in a 

 similar manner which occurs at after-periods, physiologists have 

 been naturally led to look to the ovum for the original source 

 to which all succeeding cells in the economy might be traced 

 back ; and, as that body is itself derived from the parent organ- 

 ism, it is conceived that a peculiar germinative matter is handed 

 down from parent to offspring, and, receiving an impulse by 

 fecundation, begins in the ovum the series of assimilative and 

 reproductive actions which is afterwards continued throughout 

 life. Seeing, moreover, that throughout all these operations the 

 nucleus takes the first apparent step in the propagation of cells, 

 and by itself increases and multiplies, that body may not un- 

 reasonably be regarded as the original depositary and sub- 

 sequent representative of the germinative matter in its 



most characteristic type, although, no doubt, the protoplasm of the primary cell- 

 contents is, in its degree, endowed with the same property. According to this view, 

 all cells in the animal economy are derived from pre-existing cells, and all are to 

 be referred back through preceding generations to the ovum. Schwann, on the other 

 hand, maintained that cells may arise altogether independently of pre-existing cells, 

 and that in animals this was actually the most prevalent mode of cell-production. 

 He conceived that cells are formed out of a soft or liquid organisable matter, which 

 he named " cytoblastema," or simply " blastema," and which in animals that have a 

 circulating blood is derived from that source. The blastema might be contained in 

 cells, lodged in the interstices of cells and tissues, or deposited on the surface of 

 growing parts ; and cells might arise in any of these situations, viz., in previously 

 existing cells, or in the interstitial and free blastema. As to the steps of the process 

 of formation, Schwann adopted and applied to animal cells the account given by 

 Schleiden of cell-genesis in vegetables. According to Schleiden, nuclei are first 

 formed by aggregation of matter round nucleoli which appear in the cytoblastema, 

 and a cell-wall is produced by deposition and condensation of fresh matter on the 

 nucleus, which he regarded as the generator of the cell, and therefore named the 

 " cytoblast." Among later observers who deny the uninterrupted descent of all cells 

 from the ovum, may be mentioned M. Robin, who believes that the embryonic cells 

 derived from the ovum do not generate secondary cells, but suffer complete dissolu- 

 tion, at the same time that fresh nuclei are independently formed in their insterstices 

 from the dissolved substance. 



The doctrine of independent cell-formation, which, in contradistinction to derivation 

 from previously existing parent-cells, it has been proposed to call " equivocal cell- 

 generation," has greatly lost ground since it was first promulgated ; and perhaps it 

 must be abandoned as regards cell-formation in the higher animals. At the same 

 time, undue weight must not be allowed to the natural d priori argument against it 

 as a case of spontaneous generation. It must further be borne in mind that in 

 certain lichens, algae and fungi, spores entirely constituted as cells arise, not indeed 

 independently of a parent organism, but to all appearance not immediately from pre- 

 existing cells or nuclei. 



Motion of the Protoplasm in Cells. In the cells of the Vallisneria, Chara, 

 and various other plants, when exposed under the microscope, the green 



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