244? Dll. BEALE, ON THE TISSUES. 



the following extract : — " The endoplast grows and divides ; 

 but, except in a few more or less doubtful cases, it would 

 seem to undergo no other morphological change. It fre- 

 quently disappears altogether ; but, as a rule, it undergoes 

 neither chemical nor morphological metamorphosis. So far 

 from being the centre of activity of the vital actions, it 

 would appear much rather to be the less important histologi- 

 cal element, 



*' The periplast, on the other hand, under the names of 

 cell-wall, contents, and intercellular substance, is the subject 

 of all the most important metamorphic processes, whether 

 morphological or chemical, in the animal and in the plant. 

 By its differentiation, every variety of tissue is produced ; and 

 this differentiation is the result, not of any metabolic action 

 of the endoplast, which has frequently disappeared before the 

 metamorphosis begins, but of intimate molecular changes in 

 its substance which take place nnder the guidance of the ' vis 

 essentialis,' or, to use a strictly positive phrase, occur in a 

 definite order, we know not why." 



Virchow, on the other hand, attaches the greatest import- 

 ance to cells, which always come from cells, but believes, 

 nevertheless, that " It is not the constituents w^hich w^e have 

 hitherto considered (membrane and nucleus), but the con- 

 tents (or else the masses of matter deposited without the cell, 

 intercellular), which give rise to the functional (physiological) 

 differences of tissues." The cell is " a simple, homogeneous, 

 and very monotonous structure, recurring with extraordinary 

 constancy in living organisms." It is the other contents, 

 not the nucleus or membrane, which occasion the physiologi- 

 cal action of parts. Virchow considers that the nucleus is 

 concerned in maintaining and multiplying living parts, and 

 that while fulfilling its functions it remains itself unchanged. 



Dr. Hughes Bennett, of Edinburgh, holds, on the con- 

 trary, that cells can grow fi-om a clear exudation; and he 

 considers that granules first make their appearance, and that 

 a cell-wall is afterwards formed around these. 



It is very difficult to express briefly the differences and 

 resemblances between all these conflicting views ; and it 

 would be quite out of place, in a course like the present, to 

 show in detail the several points in which the author agreed 

 with or differed from, those who had written before him. 



The author^s conclusions did not permit him to agree with 

 any of these theories. He had already alluded to the diffi- 

 culty of demonstrating the existence of a cell- wall, and had 

 shown that this is not a constant structui'e. So far from 

 regaixling the intercellular substance as the seat of essential 



