82 



THE EVOLUTION OF SEX. 



tissues to the cellular type, and traced back the plant-embryo to a 

 single nucleated cell; while, in the following year, Schwann boldly 

 extended this conception of plant structure and development to the 

 animal world, and so fully constituted the " cell-theory." The ovum, 

 recognized as a cell, became a " primordium commune" in a deeper 

 sense than Harvey dreamt of; the masses described by Prevost and 

 Dumas were seen as the products of cell-division; and Kblliker led 

 the way, now so well followed up, in tracing these cells to their results 

 in the tissues of the organism. 



IV. Protoplasmic Basis. — Only one step further is it possible 

 for biological analysis to penetrate, and that within the last few years 

 is being persistently essayed. It is impossible to rest at the cell-theory 



Fig. 20. — Ground-plan of Protoplasmic Changes. 



level. To recognize the ovum as a cell and the spermatozoan as 

 another, to find the starting-point of the organism in the double 

 unity formed from these two, to demonstrate the process of develop- 

 ment as one of cell-multiplication and arrangement, express great but 

 not final biological facts. Thus it is that of late years, what Michael 

 Foster has called the "protoplasmic movement" has made itself felt, 

 not only in study of the general functions of the body, but in the 

 special physiology of the reproductive cells and their history. Even 

 in morphological or structural studies, attention has shifted from the 

 shapes of cells to the structure of their living matter, or from the 

 different forms of ovum and spermatozoan to the germinal protoplasm 

 or keimplasma which they contain. On this level — in fact where 

 biology has touched the bottom — morphology and physiology have 



