86 THE EVOLUTION OF SEX. 



than that, he followed " the disposition of these primitive 

 elements to the upbuilding of some of the important organs. 

 He undoubtedly reached too far in his emphasis on the entire 

 simplicity of the germ, and many of his details were mistaken ; 

 but none the less did he recall embryologists from speculation 

 to take the facts as they found them, and lay the foundation of 

 modern embryology in the fact that organisation was gradually 

 acquired by an observable process of development. 



(<?) Wolff's Successors.— ^\v^ important conclusion reached 

 by Wolff remained for about sixty years without effect. In i8i 7, 

 Christian Pander took up embryological research exactly where 

 Wolff had left it, and worked out the history of the chick in 

 more exact detail. In 1824, Prevost and Dumas noticed the 

 division of the ovum into masses ; and in the following year 

 Purkinje discovered the nucleus or "germinal vesicle." Von 

 Baer followed up his friend Pander's work, and in 1827 made 

 the memorable discovery of the mammalian ovum, which he 

 traced from uterus to oviduct, and then to its position in the 

 ovary itself. Thus, after a century and a half, De Graafs 

 endeavour was at length fulfilled. Soon afterwards, Wagner, 

 von Siebold, and others, elucidated what was still hidden from 

 von Baer, — the real nature of the spermatozoa. Meanwhile, 

 Bichat's analysis (1801) of the organism into tissues, was with 

 improved appliances deepened in the casual description of 

 "cells"; and an important generalisation had its forecast in 

 1835, when Johannes Miiller pointed out in the vertebrate 

 notochord the existence of cells resembling those of plants. 



>5 3. The Cell-Theory. — Without continuing the history 

 further, we must simply note that in 1838 Schleiden referred 

 all vegetable 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 struc- 

 ture and development to the animal world, and so fully consti- 

 tuted the "cell-theory." The ovum, recognised 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 Kolliker led the 

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

 results in the tissues of the organism. 



§ 4. 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 



