l$0 THE EVOLUTION OF MAN. 



its minute and complex structure, is in form only a single- 

 celL 



In contrast to this highly complex specialized mind- 

 cell (Fig. 7) is the egg-cell (Fig. 1), which is in no way 

 specialized. Yet here, also, we are obliged to infer from its 

 active properties a highly complex chemical composition of 

 its protoplasmic substance, and a minute molecular struc- 

 ture, which are completely hidden from our eyes. 



The description of these cells as elementary organisms, 

 or individuals of the first order, must be somewhat qualified. 

 For cells by no means represent quite the lowest grade of 

 organic individuality, as that is usually understood. There 

 are yet more simple elementary organisms at which we 

 will now give a passing glance, in order to return to 

 them hereafter. These are cytods : living, independent 

 existences which consist merely of an atom of plasson ; in 

 other words, of an entirely homogeneous atom of an albu- 

 minous substance, which is not yet differentiated into 

 nucleus and protoplasm, but exercises the properties of both 

 united. For example, the remarkable Monera are cytods 

 of this kind. (Cf. Chapter XVI.) Strictly speaking, we 

 should say : the elementary organism, or the individual of 

 the first order, occurs in two different grades. The first and 

 lowest is the cytod, wfyich consists merely of an atom of 

 simple plasson. The second and higher grade is the cell, 

 which has been differentiated into nucleus and protoplasm. 

 Both grades, cytods and cells, are grouped together under 

 the idea of sculptors or builders, because they alone in 

 reality build the organism. 42 But in higher animals and 

 plants, such cytods do not, as a rule, appear, so that only 

 actual nucleated cells occur. Here, therefore, the elementary 



