268 THE CELL-THEORY 



' 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 frequently disappears altogether ; but, as a rule, it undergoes 

 I neither chemical nor morphological metamorphosis. So far from 

 I being the centre of activity of the vital actions, it would appear much 

 rather to be the less important histological element. 



The periplast, on the other hand, which has hitherto passed 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 differen- 

 tiation 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 

 under the guidance of the " vis essentialis," or, to use a strictly positive 

 phrase, occur in a definite order, we know not why. 



The metamorphoses of the periplastic substance are twofold — 

 •chemical and structural. The former may be of the nature either of 

 ■conversion : change of cellulose into xylogen, intercellular substance, 

 &c., of the indifferent tissue of embryos into collagen, chondrin, &c. ; 

 or of deposit : as of silica in plants, of calcareous salts in animals. 



The structural metamorphoses, again, are of two kinds — vacuolationt 

 or the formation of cavities ; as in the intercellular passages of plants, 

 the first vascular canals of animals ; and fibrillation, or the develop- 

 ment of a tendency to break up in certain definite lines rather than in 

 others, a peculiar modification of the cohesive forces of the tissue, 

 such as we have in connective tissue, in muscle, and in the " secondary 

 deposits " of the vegetable cell. 



Now to illustrate and explain these views, let us return to the 

 vegetable and animal tissues, as we left them in describing the base of 

 the Sphagnum leaf and fcetal cartilage, and trace out the modification 

 of these, which are identical with all young tissues, into some of the 

 typical adult forms. 



The point of the Sphagnum leaf is older than the base, and it is 

 easy to trace every stage from the youngest to the complete forms in 

 this direction. At the base of the leaf we find, as has been said, 

 nothing but minute endoplasts, each resembling the other, embedded 

 in a homogeneous periplastic substance (A) ; as we trace these upwards, 

 we find that some of the endoplasts increase in size more rapidly than 

 the others (B), and eventually totally disappear, leaving only the 

 endoplastic cavity, or "cell," which contained them. In the surrounding 

 cells, the endoplasts are very obvious as granular primordial utricles 



