58 GENERAL PRINCIPLES OF ZOO LOO Y. 



development of every animal. Embryologically every organism 

 is at some time a simple element, a cell ; this divides and gives rise 

 to tissues; from the tissues are formed organs, and from the 

 organs the regularly membered whole of the animal body is com- 

 bined. If the general ontogeny proceeds synthetically, it then 

 agrees in its manifestations with the processes which go on in 

 nature and which are accessible to direct observation. 



GENERAL ANATOMY. 



The Morphological Units. The expression < constituent parts 

 of the animal body ' can be used in a double sense. We can speak 

 of the chemical units, the chemical combinations, which form the 

 tissues; these are the subject of animal chemistry, and may there- 

 fore be passed over here. But we may also speak of the constituent 

 units (morphological units) of the animal body ; these are the cells. 

 These and their transformation into tissues, organs, and entire 

 animals are for us of vastly greater importance. 



I. THE MORPHOLOGICAL UNITS OF THE ANIMAL BODY. 



The Cell. The study of the morphological units of the organic 

 body first found a firm foundation in the cell theory. Every 

 scientific study of the anatomy of plants and animals must there- 

 fore take the cell as its starting-point. 



History of the Cell Theory. The conception of the cell of animals and 

 plants has in the course of time undergone many changes, which must 

 be known to some extent in order to understand completely the name and 

 the conception. When, in the seventeenth century, Hooker, Marcello 

 Malpighi, and Nehemia Grew introduced the term into vegetable anatomy, 

 they meant small chambers surrounded by firm walls and filled with air 

 or fluid contents. When, also, early in the nineteenth century, it was cor- 

 rectly recognized that the cell is the anatomical and physiological vegetable 

 unit from which all the other parts of the plant are formed, and when the 

 English botanist Brown discovered in the interior of the cell that small 

 body previously overlooked, the kernel or nucleus, the old conception 

 remained, and as such was accepted by Schleiden in his cell theory. 

 Schleiden added as new a completely erroneous view of the origin of cells: 

 that in a sort of matrix (the * cyroblast ') first a granule, the nuclear 

 body, was formed, then around this granule a membrane, the nuclear mem- 

 brane, arose by precipitation, and around the thus completed nucleus a 

 larger membrane (the cell membrane) was deposited. Hence for the 

 formation of the cell the nucleus would be of most importance. 



The Schleiden-Schwann Cell Theory. Since it is the nuclei which are 

 most easily seen in the animal body, and even now are particularly useful 



