THE CELL DOCTRINE. 51 



duced; and this differentiation is the result, not of 

 any metabolic action of the endoplast, 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 order, 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, &c., of the indifferent tissues of embryos into 

 collagen, chondrin, &c., or of deposit, as of silica 

 in plants, of calcareous salts in animals. The struc- 

 tural metamorphoses, again, are of two kinds, vacuola- 

 tion or the formation of cavities, as in the intercellu- 

 lar passages of plants, the first vascular canals of 

 animals and fibrillation, or the development of a 

 tendency to break up in certain definite lines rather 

 than in others." 



These views he illustrates by examples from vege- 

 table life in the sphagnum leaf, and from animal life 

 in connective tissue and striped muscle. 



As characteristic and distinguishing features of 

 this theory, we desire to point out, first, the substitu- 

 tion of the term "endoplast" for "nucleus;" that 

 of "periplast" for "cell wall," and "intercellular," 

 "substance." Second, the absolutely passive na- 

 ture of the "endoplast," which is neither itself the 

 author of changes, nor the subject of changes. 

 Third, the passive nature as well, of the " periplast," 

 so far as it is the author of changes, though it is pre- 



