80 ON THE DEVELOPMENT OF THE ANIMAL TISSUES. 



The following are the more important discoveries of Schwann,* res- 

 pecting the cells of animals, and the agreement of animals and plants 

 in their ultimate structure. 



In the chorda dorsalis, the cellular structure of which I had myself 

 pointed out long since, Schwann first discovered the nuclei or cytoblasts. 

 Each cell of the chorda dorsalis of Pelobates fuscus has its disc-like 

 cytoblast lying at the inner surface of the wall of the cell ; and in this 

 nucleus there is seen one, rarely two or three, clearly defined spots. In 

 the cavity of the cells young cells are developed, as in plants. 



Cartilages also are, according to Schwann's observations, composed 

 entirely of cells, when first formed. The cartilaginous branchial rays 

 of fishes at their apex are composed of small polyhedral cells, lying in 

 close contact with each other, and having very thin walls. These cells 

 have rounded granular nuclei. Towards the middle of the branchial 

 ray, the septa between the cavities of the different cells formed by their 

 walls, gradually become thicker. Nearer to the root or base of the 

 branchial ray, the walls of the contiguous cells can no longer be distin- 

 guished from each other, and the mass appears to be formed of a homo- 

 geneous substance containing small cavities ; but around some of the 

 cavities a circular line can be distinguished, which indicates the boun- 

 dary of the wall of the cell, and proves that the whole mass is not 

 formed by the thickened walls of the cells, but that a real intercellular 

 substance also exists. Even while the walls of the cells are still in 

 contact with each other this intercellular substance is present, at 

 that time appearing here and there like a triangular space between three 

 contiguous cells. In this form of cartilage the process of development 

 consists partly in the thickening of the walls of the cells, and partly in 

 the production of an intercellular substance. In higher vertebrate 

 animals the thickening of the walls of the cells is not observed, and 

 the principal mass of the future cartilage appears to be formed by the 

 intercellular substance, in which the cells, with the younger cells within 

 them, are included. The development of cells in the manner of the 

 cells of plants, has been observed by Schwann in the branchial cartilages 

 of Pelobates fuscus, in which some cells contain mere nuclei ; others, 

 nuclei with small cells developed upon them, and scarcely larger than 

 themselves ; others, again, larger fully formed cells. So that here all 



* Froriep's Notizen, 1838, Nos. 91, 102, 112. Schwann Microscopische Unter- 

 suchungen iiber die Ueber-einstimmung in der Structur und dem Wachsthum der 

 Thiere und Pflanzen, Berlin, 1838. [A review of this work, with a copious abstract 

 of its contents, is contained in the 9th volume of the British and Foreign Medical 

 Review.] 



