2 INTRODUCTION 



nutritive matter, called food-vacuoles, and others called water-vacuoles. 

 The ameba itself is surrounded with protoplasm (ectoplasm), which, by 

 alternate extension and contraction of processes catted pseudopods, 

 enables it to move slowly from place to place. These creeping move- 

 ments are observed in some animal cells and are called ameboid. 



Protoplasm. The various substances known collectively as proto- 

 plasm constitute the basis of life, both vegetable and animal. It is 

 composed mainly of carbon, hydrogen, oxygen and nitrogen; but the 

 composition of its molecule is constantly changing, and consequently 

 it has no fixed and definite chemical composition. In this respect it is 

 in a condition known as labile. In studying the structure and composi- 

 tion of animal cells, it will be seen that the general term " protoplasm," 

 aside from the so-called proteids, includes many substances known 

 under different names intended to express, more or less accurately, 

 different properties and relations. The typical animal cell, indeed, is 

 composed of many of these substances called by different names. 



The Typical Animal Cell. 1 Figure 2 represents diagrammatically an 

 animal cell. This is somewhat more complex in its structure than the 

 ameba. Its basis is a clear protoplasm (hyaloplasm) contained in the 

 meshes of a reticulum (spongioplasm) with minute granules (micro- 

 somes), the whole being called cytoplasm. In the cytoplasm are little 

 globules (plastids) and usually, also, dark granules or globules of vari- 

 able size (metaplasm). The most important structure in the cytoplasm is 

 the "attraction-sphere," containing two little bodies called centrosomes. 

 The cell itself is surrounded with a delicate membrane composed of 

 condensed hyaloplasm. In the cell, near its centre, is a rounded 

 nucleus with a highly refracting nucleolus, the nucleus being sur- 

 rounded with a membrane and presenting a very fine reticulum formed 

 of " linin " fibres, 2 that are not seen, however, unless brought out by 

 reagents. The nucleus also contains relatively coarse threads of chro- 

 matin, a substance chemically identical with nuclein. The substance 

 of the nucleus is sometimes spoken of as the linin. A collection of 

 chromatin also is observed in the nucleus, called the karyosome. The 

 substance of the nucleus is called karyoplasm, or linin. The cell itself 



1 The apparent logical necessity of studying the cell as an introduction to human physiology 

 is, perhaps, to be regretted for one reason; and that is a redundancy of nomenclature, always 

 somewhat confusing. Wilson, in his work on The Cell, gives a glossary with two hundred 

 different names that have been applied to parts of the cell and its processes of origin, develop- 

 ment and multiplication, only seventeen of which are classed as obsolete. I have striven, 

 however, to minimize this difficulty by adopting, so far as possible, only names that are in com- 

 mon use and to be found in modern works on anatomy. 



2 Linin-elements may be seen in cells fixed with Flemming's solution and stained with 

 picrocarmin. Poljakoff, "Biologic der Zelle," Archiv fur mikroscopische Anatomie, Bonn, 

 1900, Bd. LVI, S. 651 et seq. 



