34 THE GENERAL CHARACTERS OF CELLS, BY S. STRICKER. 



Schwann, speaking of the origin of cells, remarks, " We have, in 

 the first instance, a structureless substance, which, according to 

 its chemical qualities and the grade of its vitality, possesses 

 a greater or less capacity of effecting the development of 

 cells." 



Schwann was of opinion that the extracellular formation of 

 cells, that is, their development in free blastema, was the most 

 frequent mode of their production in animals. But the 

 experience of embryologists was soon found to be in opposition 

 to his views. The segmentation of the egg of the frog, already 

 observed in 1824 by Prevost and Dumas, led to the state- 

 ment made in the beginning of 1840, that the segments into 

 which the egg breaks up are cells. This view was in the first 

 instance defended by Eeichert,* who believed that he was 

 able to perceive a cell membrane in the several segments. Berg- 

 man~f* raised very solid objections to the existence of a membrane 

 in this instance ; and he quite correctly maintained that the 

 spheres of segmentation are cells which are at first destitute of a 

 cell wall, though they become invested by one at a subsequent 

 period. 



Henle also held that a close relation existed between the 

 process of segmentation in the egg and the division of cells, 

 and Kolliker interpreted the segmentation of the germ 

 of cephalopods in the same manner. But this view of 

 cell genesis was again very generally departed from. The 

 merit of having defended it effectually must be ascribed to 

 RemakiJ: in particular, who has chiefly contributed to the 

 abandonment of Schwann's doctrine of cell formation. Remak 

 maintained with great steadiness that in the early stages of 

 the development of the embryo no other mode of cell develop- 

 ment occurs than by division. 



To Remak also the merit is due of having established the 

 same law in respect to the pathological development of cells. 

 There is at the same time no doubt that Virchow played an im- 

 portant part in the extension of our knowledge in this direction, 



* Entwickelungsgeschichte im Wirlelthiere, 1840. 



f Miiller's Archiv, 1841. 



} Entwickelungsgeschichte. Berlin, 18521855. 



