THE CELL DOCTRINE. 39 



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, the 



Fig. s. 



From the point of a Branchial Cartilage of Kana esculenta. 

 (From Schwann.) 



nucleus may remain solid ; but if it go on more vigor- 

 ously 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 naeleus 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, lohich 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 honfogeneous, sometimes granulous: the 

 latter is most frequently the case in the thick strata 

 which occur in the formation of the rnajority of ani- 



* Schwann, op, citat., p. 175. 



