CONNECTIVE TISSUES CELLS. 59 



pores, as in the cells of the chorda dorsalis ( W. Miiller), or is deficient at 

 some part of the surface, as in the cells covering the intestinal villi. It is 

 sometimes prolonged in the form of cilia, or of processes rendering it more 

 or less stellate in form. The nucleus is a minute oval body, situated either 

 near the centre of the cell or in contact with the cell-wall, and never absent 

 in the higher animals in the first stages of the development of the cell. 

 Originally soft, it becomes more consistent with age, and appears to be of an 

 albuminous nature, resisting the action of acids and alkalies better than the 

 protoplasmic contents of cells generally. Like these, however, it stains 

 with carmine, and hence is regarded by Beale as germinal matter a view 

 which is supported by the fact that it appears to be specially related to the 

 formative activity of the cell, the division of the nucleus in most instances 

 preceding that of the cell itself. The nucleus often contains one or more 

 extremely minute particles termed nucleoli ; and Rollett, who considers the 

 nuclei to be vesicular, has described certain lacuuse or vacuolye in their in- 

 terior. The cell-contents, apart from the nucleus, whilst consisting essentially 

 of protoplasm, mingled in young cells with cholesteriu and lecithin, and in 

 many cases also with glycogen, vary as they advance to maturity with the 

 special tissue or organ of which they form a part ; being sometimes trans- 

 parent and colorless, sometimes charged with pigment, sometimes containing 

 the special secretion of a gland ; whilst at others the protoplasm becomes 

 almost wholly converted into fat, or acquires the highly differentiated prop- 

 erties of nerve and muscle. The mode of origin or genesis of cells has been 

 the subject of much controversy, and takes place in several ways. One of 

 the commonest methods is that in which the contents of a cell divide, whilst 

 the cell-wall remains unaltered till it is burst asunder by the growth of the 

 new cells in its interior. In this endogenous mode of cell genesis, which 

 occurs in fat-cells (Fig. 28), and in cartilage, the nucleus usually first 

 divides, and the two halves, or if there are more divisions, the several seg- 

 ments, retreat from each other to the opposite ends of the cell. The proto- 

 plasmic contents then undergo a process of differentiation, accumulating to 

 form separate masses around the segments of the nucleus, and each acquir- 

 ing a cell-wall ; the whole being ultimately set free by the rupture of the 

 membrane of the parent cell. In other instances the parent cell divides as 

 a whole (fission^, the cell-wall becoming inflected. The bridge between the 

 two parts then becomes gradually attenuated, and the parts at length sepa- 

 rate. The white corpuscles of the blood have been seen to divide in this 

 manner. A third mode is that of gemmation, or budding, in which a swell- 

 ing appears on one or more parts of the circumference of a cell, and grad- 

 ually increasing is nipped off by the constriction of the base. This is well 

 seen in the yeast fungus. Lastly, cells may develop independently in a 

 blastema that has been formed by pre-existing cells, free-cell formation. 

 Broca, Robin, Bennett, and Bastian are strong supporters of this view, 1 and 

 it seems to be certain that in the Myxomycetse, 2 spore-cells make their ap- 

 pearance in the blastema-like mass of which the whole organism is composed 

 without the previous existence of cells. In 1867 Legros and Onimus, in 

 their experiments on the generation of leucocytes, 3 pointed out that if little 

 sacs of goldbeaters' skin, filled with the serum of blisters, filtered to re- 

 move all morphological elements, be placed beneath the skin of rabbits, a 

 large number of leucocytes appear in the fluid in the course of twenty-four 

 hours, which, they were of opinion, must have been formed in the fluid. 



1 See Bennett's Text Book of Physiology. Bastian, Beginnings of Life, 1872, 

 vol. i, p. 169. Robin, Anat. et Physiologie Ccllulaires, 1873. Broca, Traite des 

 Tumeurs. 



2 Do Bary, Die M ycetozocn, 1864. 3 Journal do la Anatomic, 1867. 



