34 THE CELL DOCTRINE. 



the stratum becomes defined externally, and a cell 

 nucleus having a more or less sharp contour is formed. 

 The nucleus grows by a continuous deposition of 

 new molecules between those already existing, that 

 is by intussusception. (See Fig. 3, e.) If this go on 

 equally throughout the entire thickness of the stratum, 



FIG. 3. 



FIG. 3. From the point of abranchial cartilage of Rana esculenta. 

 (From Schwann.) 



the nucleus may remain solid; but if it go on more 

 vigorously in the external part, the latter will become 

 more dense, and may become hardened into a mem- 

 brane, and such are the hollow nuclei."* 



When the nucleus has reached a certain stage of 

 development, the cell is formed around it. The fol- 

 lowing is the process by which this takes place : 

 "A stratum of substance, which differs from the cyto- 

 blastema, is deposited upon the exterior of the nu- 

 cleus. (See Fig. 3, d.) In the first instance, this 

 stratum is not sharply defined externally, but be- 

 comes so in consequence of the progressive deposition 

 of new molecules. The stratum is more or less thick, 

 sometimes homogeneous, sometimes granulous: the 



* Schwann, op. citat., p. 176. 



