Structure and Growth in Anhnah and Plants. 29 



^orbed, and a network of canals is formed of a very unequal thickness. 

 The contents of the primary cells, secondary or composite, is the san- 

 guineous liquid. (According to Valentin, the internal membrane of the 

 capillary vessels is produced from elongated or ramified cells. The ex- 

 terior fibres, as well as the filiform epithelium, originate from the cellular 

 fibres, which are formed and collected in masses on the exterior.) 



Valentin distinguishes in the formation of tissues a secondary deposit 

 of enveloping substance ; it is observable in the ganglionic globules of the 

 brain and nerves. The primary cell is produced along with its nucleus^ 

 and then itself performs the function of a nucleus, so that its nucleus be- 

 comes the nucleolule, and the primitive nucleolules become nucleolules 

 of secondary power. Around the cell is disposed a mass of grains, united 

 by a transparent matter, and surrounded by a simple cellular membrane. 

 In the egg, we see'new cells developed in the midst of the enveloping 

 mass, which determine the formation of the vitelline globules, and other 

 cells of still higher importance, and which, by their metamorphosis, have 

 a direct influence on the development of the parts of the embryo. What 

 takes place in the cellular formation of the first degree, the deposition 

 of a heterogeneous mass around the nucleus, is reproduced in the second 

 degree, in the ganglionic globules and in the egg. For other details, re- 

 ference may be made to Valentin on the development of tissues (i2. Wag- 

 ner, Physiologic, 1339, 132.) 



We shall give, in conclusion, and always according to J. Miiller, a 

 sketch of the principal results which Schwann has arrived at. 



The most different elementary parts of animals and plants have a com- 

 mon mode of development ; their origin is always a cell. We first see a 

 substance presenting no structure, and which is found either in the inte- 

 rior of the cells, or between the cells already existing. It is in this sub- 

 stance that cells are formed, according to determinate laws, and these 

 cells are developed in different ways in order to form the elementary parts 

 of the organisms. In each tissue no new cells are formed, except in the 

 points to which new nutritive elements penetrate ; whence the difference 

 between the tissues which contain vessels and those deprived of them. In 

 the first, the nutritive fluid spreads in every direction ; here the new cells 

 appear in all the thickness of the tissue. In certain tissues without ves- 

 sels, the nutritive fluid is brought only to the inferior, that is, the internal 

 or adherent face, as takes place with an epidermis. In cartilages, when 

 they are still without vessels, new cartilaginous cells appear only at the 

 surface, and then arrange themselves in a circle. 



The expression, increase hy juxtaposition, is good when used to express 

 the production of new cells, and not the growth of such as exist. The 

 new cells of the epidermis appear only beneath the precediDg. 



In both these cases the cells grow by intro-susception. 



Bones are found, up to a certain point, in a mixed state. The cartilage 

 is at first without vessels, and the new cells are formed near* its exterior 

 surface. When vessels are developed in the medullary canals, the forma- 

 tion of a new cytobl«^t and new cells may take place in p.irt at the sur- 



