18 ANIMAL PHYSIOLOGY. 



or not. The membrane, when present, is called a cell-wall^ 

 and the structure of which it is the limit is a nucleated cell ; 

 and in consequence of the circumstance that the mass of pro- 

 toplasm was the last part of the corpuscle to have due atten- 

 tion attracted to it, and that the outline of the corpuscle was 

 often mistaken for a membrane, even when no membrane 

 existed, the importance of the cell-wall was formerly over- 

 estimated, and the word cell is even yet often used to indicate 

 structures without a cell-wall, which are better designated as 

 corpuscles. The cell-wall is probably in all instances a deposit 

 round a pre-existing corpuscle. 



At an early period of embryonic existence, the body may 

 be said to consist entirely of nucleated corpuscles; and even 

 after birth, the younger the animal the more abundant are 

 these elements in the textures, and the more easily exhibited 

 under the microscope. They are found in numbers wherever 

 there is much growth; and in rapidly increasing tumours 

 they exist in greatest plenty. They are also the germs from 

 which the more complex elements of texture take origin. 

 Thus nerves and voluntary muscular fibres originate by 

 metamorphosis of nucleated corpuscles, which, in becoming 

 more highly developed, lose the reproductive power, while 

 they gain, in the one case, nervous activity, and in the other 

 greatly increased contractility. Both of these tissues in early 

 development present long bands of albuminoid substance, 

 with a row of nuclei in each. 



B 



Fig. 3. MULTIPLICATION OF NUCLEATED CORPUSCLES. A, Corpuscles 

 from connective tissue of a foetal lamb, some of them dividing. B, 

 Endogenous multiplication within a brood cell from a tumour. 



Nucleated corpuscles multiply by division, which is termed 

 fissiparous when the parts into which they divide are of 



