ON THE DEVELOPMENT OF THE ANIMAL TISSUES. 81 



the stages of the development of a cell are present. The process of the 

 development of cartilage seems to be independent of blood-vessels, and 

 to be wholly analogous to the process of growth in vegetable tissues : 

 how the canals radiating from the corpuscles of ossified bone are deve- 

 loped is not known. Two hypotheses are proposed by Schwann. If 

 the osseous corpuscles are the cavities of cells, the thickened walls of 

 which have coalesced with each other and with the intercellular sub- 

 stance, so as to form the mass of the cartilage of the bone, then the 

 radiating canaliculi may be regarded as canals extending from the cavi- 

 ties of the cells through their thickened walls, and would be analogous 

 to the pore-like canals of some vegetable cells. But if the osseous 

 corpuscles are the cells themselves, and not merely their cavities, the 

 whole substance between the corpuscles being intercellular substance ; 

 in this case the canaliculi will probably be radiating prolongations of 

 the cells extending into the intercellular substance. According to the 

 latter view, which Schwann regards as the more probable, the canaliculi 

 would correspond to the processes given off from some cells of plants. 



Besides the formation of young cells in the cavities of previously 

 existing cells, Schwann has observed their development in the exterior 

 of other cells in a structureless substance, the cytoblastema. In this 

 case, also, the nucleus generally appears to be first formed, and the 

 cell to be afterwards developed around it. In many animal tissues, the 

 new cells are formed on the exterior of the earlier cells. In the one 

 case the cytoblastema exists in the interior of the cells already existing; 

 in the other it is external to them. 



Schwann arranges the tissues of the animal organism, according to 

 the mode of their development, in five classes : 



I. Isolated independent cells, which either float free in a fluid, or 

 if deposited, in contact with each other, are still unconnected and 

 moveable. 



II. Independent cells, arranged so as to form a continuous mem- 

 brane. 



III. Tissues formed of cells, the walls of which have coalesced, while 

 their cavities remain distinct. 



IV. Fibre cells. Cells which have become elongated in different 

 directions, and resolved into bundles of fibres. 



V. Cells, both the walls and the cavities of which have coalesced, 

 so as to form tubes. 



To the first class belong the corpuscles of the blood. The vesicular 

 nature of these bodies was observed by C. H. Schultz. Their nucleus, 



VOL. II. G 



