ELEMENTS OF STRUCTURE. 



89 



55. 



2. Passing on now to the consideration of segmentation in cells 

 endowed with a wall or capsule, we find perhaps the best example in 

 the cellular elements of cartilage. Endogenous growth in the cells in 

 question does not take place with the same simplicity as in the process 

 just referred to, and is a phenomenon the details of which are not yet 

 fully known; so that in the following description much that is hypothe- 

 tical must be advanced to supply points upon which we possess no certain 

 information yet (fig. 88). 



The nuclei of naked cells enveloped in secondary capsules (b) are found 

 at first to possess a single nucleolus (1). On the commencement of the 

 process of division this becomes double (2); upon which a transverse 

 furrow may be recognised in the nucleus (3) ; by the deepening of this 

 furrow the latter is eventually divided into two segments (4); which 

 then recede from one another, thus initiating the constriction of the cell- 

 body (5) ; on the completion of the latter act (6), two perfectly distinct 

 cells (7) are found within the capsule, which has remained throughout 

 entirely passive. These new elements are known as " daughter cells," 

 while the original cell, or, more correctly speaking, the capsular membrane 

 of the same, has received the inappropriate name of " mother cell," or 

 parent cell. 



Now, if this sketch be correct, the only difference existing between the 

 simple division described afc (1), and the latter, consists in the presence 

 of a capsule, so that in a blood-corpuscle of a mammalian embryo, if we 

 imagine it endowed with an envelope, we have precisely the same plan of 

 segmentation as that of cartilage cells. 



But the division of the latter does not by any means always stop here ; 

 in both daughter cells the 

 same process of segmentation 

 may be repeated until the 

 capsule includes four of them 

 (8), around which capsular 

 formations are eventually de- 

 veloped (e). So by a repe- 

 tition over and over again of 

 the same acts, whole genera- 

 tions of new cells may be 

 produced within a common 

 capsule (9). 



On the fusion of this 

 envelope of the parent cell 

 with the surrounding in- 

 tercellular substance, the 

 daughter cells may eventually 

 appear to lie free in the 

 matrix. Part of the cartil- 

 age cells which have multi- 

 plied on the above plan assume an apparently free condition in this 

 way. Others again remain permanently enclosed in the parent capsule. 



The ovum, after fertilisation has taken place, presents to us a similar 

 process of cell-division, of the utmost anatomical and physiological moment, 

 known as yelk-segmentation (fig. 89). The mode in which this takes 



Fig. 88. Plan of dividing cartilage cells, a, oody of cells; 

 6, capsules ; c, nuclei ; d, endogenous cells ; e, subsequent 

 formation of capsules around the latter. 



