INTRODUCTION 3 



of the cellular activities involved in fertilization and growth. Wc shall 

 therefore begin with a brief account of the structure of the cell to obtain 

 an idea of the condition of the zygote from which the individual arises. 



THE CELL 



The structural unit of the animal or plant body is called the "cell." 

 The concept of the cell has undergone change since the term was first 

 used. When first introduced it was applied, as the name signifies, to the 

 wall of the little compartment containing the protoplasm. As the con- 

 tents of this "cell" were studied, it became evident that the small mass 

 of protoplasm contained a proto- 

 plasmic nucleus in which compli- 

 cated changes took place during the 

 growth of the organism ; and in time 

 the word "cell" came to mean not 

 only the wall but its entire contents. 



The Cell (Fig. 1).— The proto- 

 plasm within the cell wall consists 

 of two primary components : nucleo- 

 plasm, composing the nucleus (nu) ; 

 and the cytoplasm, or the extra- 

 nuclear portion. The nucleus, con- 

 taining nuclear sap, or karyolymph, 

 is bounded by the nuclear mem- 

 brane (nm). In the karyolymph is 

 the nuclear reticulum composed of 

 linin (Z), an achromatic supporting 

 material, and chromatin (cm). One 

 or more plasmosomes (true 

 nucleoli), a reservoir of nutritive 

 material, is usually found in the 

 nucleus (nu). In addition, karyo- 

 somes (chromatin nuclei), which are accumulations of chromatin on 

 certain points of the reticulum, may be present (k). A centrosome (ct) 

 is usually found in animal cells, occupying the center of a differentiated 

 region — the centrosphere, or attraction sphere — which at the time of cell 

 division is the focus of a system of radiating astral rays collectively 

 known as the "aster." Minute bodies having the form of granules, rods, 

 or threads, known as " chondriosomes " or "mitochondria," are present 

 in the cytoplasm, as well as metaplasmic inclusions, passive accumula- 

 tions of food materials, and differentiation products. The cell wall itself 

 is regarded by many as a secretion of the protoplast, or cell proper. 



Fig. 1. — An animal cell, (c) Chro- 

 midia. {ch) Chondriosome. (cm) Chro- 

 matin, (ct) Centrosome. (ec) Ectoplasm. 

 (en) Endoplasm. (A;) Karyosome. (l) 

 Linin. {nm) Nuclear membrane, (nu) 

 Nucleolus within the nucleus, (pi) Plastid. 

 (v) Vacuole. 



