OF ALL ANIMAL TISSUES. 39 



with separate nuclei in most of the tissues, distinct from those 

 which have cells around them, but we also find that the 

 younger the cells are, the smaller they are in proportion to 

 the nucleus. The ultimate destiny also of the nucleus is 

 similar to that of the vegetable cells. As in the last named, 

 so in most animal cells it is subsequently absorbed, and remains 

 as a permanent structure in some few only. In plants, ac- 

 cording to Schleiden, the young cells are always developed 

 within parent cells, and we have also seen such a development of 

 new cells within those already farmed in the chorda dorsalis 

 and cartilage. If, however, any doubt existed as to whether 

 the primary cells of these tissues were formed within previously 

 existing parent cells, none such can arise in reference to many 

 of the tissues next to be considered. We shall indeed fre- 

 quently meet with a formation of young cells within older 

 ones, but it is not the rule, and does not occur at all with 

 regard to many of them. 



The following admits of universal application to the forma- 

 tion of cells ; there is, in the first instance, a structureless 1 

 substance present, which is sometimes quite fluid, at others 

 more or less gelatinous. This substance possesses within 

 itself, in a greater or lesser measure according to its 

 chemical qualities and the degree of its vitality, a capacity to 

 occasion the production of cells. When this takes place the 

 nucleus usually appears to be formed first, and then the cell 

 around it. The formation of cells bears the same relation to 

 organic nature that crystallization does to inorganic. The 

 cell, when once formed, continues to grow by its own individual 

 powers, but is at the same time directed by the influence of 

 the entire organism in such manner, as the design of the 

 whole requires. This is the fundamental phenomenon of all 

 animal and vegetable vegetation. It is alike equally consistent 

 with those instances in which young cells are formed within 

 parent cells, as with those in which the formation goes on 



1 [Strukturlos. — I have ventured to translate this word as ahove, although 1 am 

 aware it is open to ohjection. The idea intended to he conveyed hy the author is 

 that of a suhstance in which no definite structure can he detected. As the word 

 will be frequently used in the following pages, the reader is requested to assign this 

 signification to it invariably. — Trans.] 



