haeckel's anthropogenie. 233 



scope) ; and that of " prsedelineation," of wliich. tlie chief champions 

 were Haller and Bonnet, was most generally received. According 

 to the most advanced form of this theory, development is nothing 

 but the growth of parts preformed at the creation, though infinitely 

 small, and the ovum contains within it the rudiments of all its future 

 progeny encased one within the other. Leeuwenhoek's discovery of 

 spermatozoids in 1690 divided the prsedelineationists into animalcu- 

 lists and ovulists, the latter having decidedly the best of it, when 

 Bonnet's observations on the parthenogenesis of Aphis were made 

 known in 1745. 



"Wolff's " Theoria Generationis " — the only true one, that of epi- 

 genesis — put forward in 1759, met only with abuse. Especially he 

 showed, in reference to the development of the intestinal canal, that 

 not a trace of it is to be found in the earliest condition of the egg ; 

 that the ovum and spermatozoid indeed are entirely different in their 

 structure from the adult. He described the embryo as a flat leaf -like 

 body, divisible into four layers, each layer being converted into a 

 tube by the convergence of its edges, and all giving rise to the four 

 great systems — nervous, muscular, vascular, and alimentary. He 

 recognized the fact that these layers were formed of ultimate vesicles 

 (cells) similar to each other. The translation of Wolff's work into 

 German, in 1812, gave a great impulse to the study of ontogenesis. 

 Wtirzburg was the seat of the most important investigations : there 

 Pander, in 1817, supplemented Wolff's theory, and described the 

 division of the germ into the serous and mucous layers. Shortly 

 afterwards Baer began his researches, and' in 1828 appeared the first 

 part of the classical work on the developmental history of animals. 

 This was followed by the second part in 1837. He showed the 

 mode of derivation of the four secondary germ-layers from the two 

 primary (animal and vegetative), and what Wolff had previously btit 

 inaccurately indicated, the mode of formation of the different systems 

 of organs from these different layers. It was he who first described 

 the human ovum as found in the interior of the Graafian follicle, 

 which had formerly been mistaken for the ovum : it was he who first 

 discovered the mode of formation of the blastoderm and of the 

 chorda dorsalis. 



Much more important than the above were his comparative obser- 

 vations, which led him to divide the animal kingdom into four groups, 

 radically differing in their types of development ; a division simul- 



