CELL-FORMATION 179 



set end to end a mistaken view soon corrected by Vogt 

 (Euibryologic des Salmoncs, p. 206, 1842). 



In this detail part of his book Schwann accumulates 

 material for a general theory of the cell which he develops 

 in the third and last section. Taking up the physiological 

 or dynamical standpoint, he points out that one process 

 is common to all growth and development of tissues 

 both in animals and plants, namely, the formation of cells, 

 a process which he conceives to take place in the 

 following manner. There is, first of all, a structureless 

 substance, the cytoblastem, the matrix in which all cells 

 originate. The cytoblastem may be either inside the cells, 

 or, more usually, in the spaces between them. It is not 

 a substance of definite chemical and physical properties, for 

 the matrix of cartilage and the plasma of the blood alike 

 come within the definition. It has largely the significance of 

 food material for the developing cells. In plants, according 

 to Schleiden, cells are never formed in the intercellular 

 substance the cytoblastem is within the cells ; but extra- 

 cellular cell formation seems to be the general rule' in animals. 

 An intracellular formation of cells occurs only in the ovum, 

 in cartilage cells and chorda cells and in a few others, 



o 



and even there it is not the exclusive method of formation ; 

 a formation of cells within cells never occurs in muscles and 

 nerves, nor in fibrous tissue (p. 204). In the cytoblastem 

 granules appear, which gradually increase in size and take on 

 the characteristic shape of nuclei ; round each of these 

 a young cell is formed. Sometimes the young cells appear 

 to have no nuclei, as in the intracellular brood of chorda 

 cells, but, as a rule, a nucleus is clearly visible. The nucleus 

 is indeed the most characteristic constituent of the cell. 

 " The most important and most constant criterion of the 

 existence of a cell is the presence or absence of the nucleus," 

 writes Schwann near the beginning of his book (p. 43). 



As a general rule the nucleolus is formed first, and round 

 it by a sort of condensation or concretion the nucleus, which 

 is frequently hollow, and round this again, by a somewhat 

 similar process, the cell. " The whole process of the 

 formation of a cell consists in the precipitation round a small 

 previously formed corpuscle (the nucleolus) of first one layer 



