THE CELL DOCTRINE. 57 



frequently disappears altogether ; but as a rule it 

 undergoes neither chemical nor morphological meta- 

 morphosis. So far from 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 name of cell-wall, contents 

 and intercellular substance, is the subject of all the 

 most important metamorphic processes, whether mor- 

 phological or chemical, in the animal and plant. By 

 its differentiation, every variety of tissue is pro- 

 duced ; and this differentiation is the result, not of 

 any metabolic action of the endoplasi, which has fre- 

 quently disappeared before the metamorphosis begins, 

 but the 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 oi'der, we know not why. 



" The metamorphoses of the periplastic substance 

 are twofold, — chemical and structural. The former 

 {chemical),- may be of the nature either of conversion, 

 — change of cellulose into xylogen, intercellular sub- 

 stance, etc., of the indifferent tissues of embryos, 

 into collagen, chondrin, etc., — or of deposit, — as of 

 silica in plants, of calcareous salts in animals. The 

 structural metamorphoses, again, are of two kinds, 

 vacuolation or the formation of cavities, as in ,the 

 intercellular passages of plants, the first vascular 

 canals of animals ; and fbriltation, or the develop- 

 ment of a tendency to break up in certain definite 

 lines rather than in others." 



These views he illustrates by examples from vege- 



