982 SPECIAL PHYSIOLOGY. 



matter. This protoplasm frequently presents movements, seen also in 

 vegetable cells, which may depend on a contractility in the protoplasm, 

 for it has been observed to be excited by electrical currents (p. 149). 

 This protoplasm usually diminishes, becomes altered, or disappears, as 

 the cell grows ; but sometimes it is retained during its whole existence. 

 It is very easily colored by carmine, and is thus made evident in mi- 

 croscopic investigations. The soft substance which gathers especially 

 around the nucleus, corresponds with the germinal matter of Beale, 

 which he regards as the formative matter of the cell, as distinguished 

 from the special contents, or investments, which he names formed mat- 

 ter. The nucleus is a vesicular body, which in a growing cell is round 

 or oval ; its nucleolus is also, by some, said to be vesicular, but it may 

 be, for a time at least, solid. These two structures are the essential 

 parts, the essential puncta of the cell, the so-called germinal centres, 

 or centres of cell nutrition and cell life. 



It is supposed by Schwann that an animal cell may arise in the 

 soft, clear substance known as blastema, a sort of germinal matter ; 

 that in this, by the development and collection of a number of minute 

 molecules, a nucleus, called by him a cytoblast, is formed ; and, lastly, 

 that upon this a fine membrane grows, and gradually separating itself 

 from the nucleus, forms the cell-wall, with its intermediate cell con- 

 tents. This mode of origin is also supposed by Schwann to be the 

 one by which new cells continue to be formed during the whole of life ; 

 but it is more commonly believed, that the nuclei of new cells proceed 

 from the division or multiplication of pre-existing cells, and that all 

 are the direct descendants of those originally formed in the ovum. 

 The hypothesis of the free formation of cells is, as regards tissue 

 life, the analogue of spontaneous generation, as regards animals them- 

 selves. 



The formation of new animal cells from pre-existing cells, may take 

 place in several ways. Thus the old or parent cell may divide into 

 two cells ; and in this case, the nucleus first separates into two, and 

 the cell itself then presents an indentation across its centre, which, 

 gradually increasing, divides it into two cells, each containing its 

 proper nucleus; or sometimes the nucleus, instead of dividing into 

 two, may divide into three, four, or, as has been noticed in the embryo 

 of the frog, even into six nuclei, the cell itself then separating into a 

 corresponding number of cells. This has been named the fasiparous 

 mode of development. In the so-called gemmiparous development, 

 new cells are described as being formed by the evolution and subse- 

 quent detachment of buds from the side of a cell. Instances occur, 

 as in the soft medullary tissue in the interior of bones, and also it is 

 said, in the spleen, of the formation, by repeated subdivision, of 

 many nuclei in the interior of a single parent cell. In bone, these 

 multiple nuclei of the cells remain as nuclei ; but in the spleen, they may 

 develop into an ordinary nucleated cell. Free nuclei are those which 

 are found in a blastema or matrix, in very actively growing parts, or 

 in morbid new growths ; they originate from, or by the influence of, 

 the old or pre-existing nuclei of the surrounding tissues, themselves 

 the progeny of still anterior cells. They may appear to be evolved, 



