GENERAL HISTOLOGY. 

 I. THE CELL. 



DURING the latter part of the seventeenth century, Hooke, Mal- 

 pighi, and Grew, making observations with the simple and imperfect 

 microscopes of their day, saw in plants small compartment-like 

 spaces, surrounded by a distinct wall and filled with air or a liquid ; 

 to these the name cell was applied. These earlier observations were 

 extended in various directions during the latter part of the seven- 

 teenth and the eighteenth century. Little advance was made, 

 however, until Robert Brown (1831) directed attention to a small 

 body found in the cell, previously mentioned by Fontana, and 

 known as the nucleus. In the nucleus Valentin observed (1836) 

 a small body known as the nucleolus. In 1838 Schleiden brought 

 forward proof to show that plants were made up wholly of cells, 

 and especially emphasized the importance of the nuclei of cells. In 

 1839 Schwann originated the theory that the animal body was 

 built up of cells resembling those described for plants. Both 

 Schleiden and Schwann defined a cell as a small vesicle, surrounded 

 by a firm membrane inclosing a fluid in which floats a nucleus. 

 This conception of the structure of the cell was destined, however, 

 to undergo important modification. In 1846 v. Mohl recognized in 

 the cell a semifluid, granular substance which he named protoplasm. 

 Other investigators (Kolliker and Bischoff) observed animal cells 

 devoid of a distinct cell membrane. Max Schultze (1861) attacked 

 vigorously the older conception of the structure of cells, proclaim- 

 ing the identity of the protoplasm in all forms of life, both plant and 

 animal, and the cell was defined as a nucleated mass of protoplasm 

 endowed with the attributes of life. In this sense the term cell is 

 now used. 



The simplest forms of animal life are organisms consisting of 

 only one cell {protozoa). Even in the development of the higher 

 animals, the first stage of development, the fertilized egg, is a single 

 cell. This by repeated division gives rise to a mass of similar cells, 

 which, owing to their likeness in shape and structure, are said to be 

 undifferentiated. As development proceeds, the cells of this mass 

 arrange themselves into three layers, the germ layers, the outer one 

 of which is the ectoderm, the middle one the mesoderm, and the inner 

 one the entoderm*. In the further development, the cells of the 

 germ layers change their form, assume new qualities, adapting 

 themselves to perform certain definite functions ; a division of labor 

 ensues, the cells become differentiated. Cells having similar shape 



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