2 INTRODUCTION TO SEXUAL PHYSIOLOGY 



it is characteristic of tlie Protozoa among animals and the Proto- 

 phyta among plants, where the entire substance of the body 

 consists of a single undifferentiated cell. The nucleus undergoes 

 division as well as the external protoplasm or cytoplasm outside 

 the nucleus, so that each product of cell division comes to contain 

 a nucleus just like the parent cell. 



The higher forms of life consist of many cells, whole groups 

 of which are separated off to subserve particular functions, and 



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W§^^ 



O.V X'^ 



Fig. 1. — Amoeba protens. x 330. (After Griiber, from Shipley and 

 MacBride's Zoologi/^ Cambridge University Press.) 



1, Nucleus; 2, contractile vacuole through which waste matter is excreted 

 in solution ; 3, pseudopodium ; 4, food vacuole ; ."), grains of sand. 



these groups are packed together in various ways to form the 

 different tissues. The body of a higher animal is, however, 

 derived from a single cell, and this in the process of individual 

 development undergoes a long series of divisions in the course of 

 which the nuclei also divide. The products of division, that is 

 to say, the cells with their contained nuclei, become gradually 

 specialised and so give rise to the tissues of various kinds — 

 bone, cartilage, muscle, nerve, skin, gland, etc. Thus the outer 

 layer becomes adapted for protection and for the reception of 

 impressions produced by changes in the surroundings ; the inner 



