ELEMENTS OF STRUCTURE. 



83 



m 



of much deeper import in histology, in which material supplied by the 

 cell-body solidifies and assumes definite forms, a process of the greatest 

 importance in histogenesis, and upon which great stress vas laid many 

 years ago, especially by Koelliker. 



The formations to which we refer may be regarded at one time as pro- 

 duced by secretions from the surface of the protoplasm ; at another by 

 metamorphosis of peripheral layers of the latter. In fact, both these 

 processes are merged into one another so frequently that no great stress 

 need be laid on their separation. 



These solid formed structures, although of greater significance in the 

 bodies of the lower animals than in the human, appear nevertheless to play 

 no unimportant part in our organisation, although the distinctions between 

 the processes by which they are formed are still obscure. 



In one of the foregoing sections the cortical layer of the protoplasm of 

 cells was described, as also the cell-wall, which was recognised as an 

 envelope differing chemically from the rest of the element. 



When such membranes attain a certain degree of thickness and inde- 

 pendence as regards the body of the cell, "they are known as cell-capsules. 



The best examples of such capsular membranes are to be found among 

 the elements of a very widely spread tissue, cartilage (fig. 72). 



The cartilage cell proper (b), consists of a nucleus (a) imbedded 

 transparent contractile protoplasm. On 

 the surface of the latter a chemically 

 different layer is gradually formed, which 

 is at first thin and delicate, but eventu- 

 ally attains considerable thickness (c) 

 by the deposit on its internal surface of 

 laminae of new matter. Not unfrequently 

 a distinctly concentric marking may be 

 observed in the capsule, as optical ex- 

 pression of this successive formation 

 within it of lamina?. Again, much may 

 be learned from the action of water upon 

 the body of the cell, under which the latter shrivels up and becomes 

 widely separated from the capsule (3). 



It is probable that the thick tough envelope known as the chorion (fig. 

 73), which invests the primitive ovum cell, is 

 of analogous nature to the cartilage capsule. 

 This has recently been discovered to possess 

 a very peculiar structure ; it is marked, namely, 

 by very delicate radiating lines, which are the 

 optical expression of extremely fine passages 

 or canaliculi, known as the "pore-canals" of 

 Leydig. These which are also present in 

 vegetable cells are undoubtedly of the deepest 

 significance in cell-life. 



As related to these capsular structures 

 enveloping whole cells, other formations may 

 be mentioned which are only partial, occurring 

 on the free surface of epithelial cells. They are to be found, for instance, 

 among the columnar epithelial cells of the mammalian intestine, with their 

 delicate pore-canals, discovered many years ago almost simultaneously 

 and independently of one another, by Funke and Koelliker. 



Fig. 72. Diagram of three cartilage cells 

 M-ith capsules, a, nucleus; 6, cell-body 

 c, capsules. 



Fig. 73. Ovum of the mole (copied 

 from Leydig). a. nucleus ; 6, 

 cell-body; r, thickened capsule 

 traversed by pores. 



