30 Microscopical J^esearches, ^-c. 



face of the bone, and in part around these fsmall medullary canals. This 

 explains the disposition of the cartilage in layers or plates concentric with 

 the surface or with the small medullary canals. 



The following, moreover, is the mode of the formation of cells : in the 

 midst of a cytoblast, without structure or finely granular, we see, after 

 some time, rounded corpuscles develope themselves ; these are the nuclei 

 around which the cells are formed. The nucleus of the cell is itself gra- 

 nular, and either filled or hollow. From the nucleus first springs the cor- 

 puscle of the nucleus ; around the latter a layer of a finely granular sub- 

 stance is deposited ; the nucleus increases. Around the nucleus the cell 

 is at last formed, by the deposition of a layer of a substance distinct from 

 the surrounding cytoblast. This layer is not very well defined at first. 

 When once the membrane of the cell has acquired consistency, it becomes 

 extended by the continual admission of new molecules between those 

 already in their place ; it is lengthened out, by this development, from 

 the primitive nucleus, whence it follows that this nucleus remains fixed 

 on a part of the internal surface of the walls of the cell. The formation 

 of cells is nothing more, therefore, than a repetition of the mode of the 

 formation of the nucleus, by means of which the latter is developed around 

 its corpuscle ; only the formative activity is greater in the development 

 of the cell than in that of the nucleus. The membrane of the cell is che- 

 mically difierent in the different kinds of cells ; and, in cells of the same 

 nature, the chemical composition varies with age. According to Schlei- 

 den, the membrane of the youngest cells of plants is soluble in water ; 

 afterwards it is not so. The contents of the cells varies still more ; fatty 

 matter, pigment, &c. In the interior of a cell, which is at first as trans- 

 parent as water, there may take place by degrees a granular precipitate 

 commencing round the nucleus ; on the other hand, the granular contents 

 of a cell may dissolve insensibly. 



It is easy to see that cells are small organs in which reside the power 

 that presides over absorption and secretion. On all absorbing surfaces we 

 find a layer of similar cells, which constitutes the epithelium ; they sur- 

 round villosities, and may be compared to the cells of the spongioles in 

 the roots of plants. In the excretory canals of glands we likewise find, 

 according to Henle and Purkinje, a layer of epithelium cells ; the whole 

 mass of the liver, and even the tissue of glands, without excretory canals 

 (thymus, &c.), are likewise formed of cells inclosing a nucleus. 



According to Schwann, all the cells exercise a remote chemical action 

 {metabolische) on the cytoblast, which determines the secretions. The 

 vessels conduct the liquid about to be modified ; the cells which compose 

 the canals of glands are the modifying elements. 



With regard to the theory of cells, of which Schwann has now laid 

 down the principles to serve as a basis to a theory of the vegetative func- 

 tions of organized beings, I refer (says J. Midler), to the work itself."^- 



* From Annates des Sciences Naturcllesf, January 1842, p. 1. 



