DEVELOPMENT AND METAMORPHOSES OF CELLS. 



115 



Fig. 20. 



Tadpole, we observe, within the large parent-cells that are held toge.ther by 

 intercellular substance, a, b, c, secondary cells in various stages of develop- 

 ment: at </, the nucleus is single ; at e it is dividing into two ; in the adjoining 

 cell, the division into two nuclei, d' and e', is complete ; at h, two such nuclei 

 are inclosed within a common cell-membrane; at i, we see three new cells 

 (one of them elongated, and itself probably about to subdivide) within the 

 parent ; and in each of the two groups at the top and bottom of the figure, 

 we have four small cells, now separated by partitions of intercellular sub- 

 stance, but having manifestly originated from one parent cell. (See also Fig. 

 43.) 



130. In other cases, the granular nucleus subdivides into a greater number 

 of parts, so as to give origin to a cluster of young cells, which may completely 

 fill the parent-cell; various stages of 



this process are seen in Fig. 20. 

 This process seems to be adopted, 

 where rapid multiplication is need- 

 ed, and where the new or secondary 

 cells are not destined to possess any 

 great duration. The same nuclei 

 or "germinal centres," continually 

 drawing new materials from the 

 blood, may thus develope many suc- 

 cessive crops of new cells, when an 

 opening in the wall of the parent- 

 cell permits them to be discharged 

 as fast as they are formed ; and this 

 we shall find to be the way in which 

 the cells of the secreting structures 

 are developed within the glandular 

 follicles. According to Dr. Barry, it 

 is not uncommon for several annuli 

 of young cells to be generated from 

 the periphery of the nucleus, and to 

 attain a certain degree of develop- 

 ment within the parent cell, the first- 

 formed being the largest, as shown 

 in Plate I., Fig. 10, a, b ; some of 

 these, moreover, having distinct nu- 

 clei of their own, from which a third 

 generation is being developed on the 

 same plan (Fig. 12, b); and yet for all these to disappear by liquefaction, 

 leaving the cavity of the parent-cell unoccupied, except by a pair of cells ori- 

 ginating in the central part of the nucleus (Fig. 13). If this account be cor- 

 rect, it is probable that these temporary cells perform the office of preparing 

 the contents of the parent-cell for the nutrition of the offspring which is to 

 succeed it ; and each of these twin cells, in its turn, going through the same 

 series of changes (Fig. 14), gives origin to a new pair; the continuance of 

 which process generates a cluster. This is the mode, according to Dr. Barry, 

 in which the first cells of the embryo are developed into the "mulberry mass" 

 (Fig. 15), by whose subsequent development and metamorphoses, the tissues 

 and organs of the foetus are progressively evolved. 



131. Notwithstanding the numerous varieties that exist, in the particular 

 modes in which the cells are developed, it seems to be well established as a 

 simple general principle, that all cells take their origin in germs prepared by 

 a previously-existing cell ; and that these germs may be developed, either 



Endogenous cell-growth in cells of a meliceritous 

 tumour; a, cells presenting nuclei in various stages 

 of development into a new generation; fc, parent- 

 cell filled with a new generation of young cells, 

 which have originated from the granules of the nu- 

 cleus. 



