132 BRITISH APHIDES. 



thought into its true channels. Even the great 

 Leibnitz in his theory of monads leant to the belief 

 that all men were contained in the sperm-cells of the 

 first man. 



This theory of " encasement " was thought by some 

 to have a certain support from Bonnet's discovery, 

 made about that time, of the parthenogenetic repro- 

 duction of Aphides. 



A wholly different view, however, was taken by C. 

 F. Wolff, who, following Harvey, and more distantly 

 Aristotle, experimentally showed that there was a 

 true conversion and assimilation of material around a 

 particular spot or germ in the egg; but his explanation 

 met with the greatest opposition; and, indeed, this 

 doctrine of epigenesis was almost ignored by anato- 

 mists for many years after the death of its propounder. 

 Haller strongly opposed this development of organs 

 around the " punctum saliens," and gave his dictum as 

 an unanswerable truth " Nulla est Epigenesis." Wolff, 

 however, may be said to be the father of the theory 

 of development from the germinal layers ; a theory 

 which Pander more fully investigated in 1817, and 

 Von Baer further expanded in 1828. 



Although the primitive germs of all animals are so 

 similar that the microscope scarcely shows a difference 

 between them, Yon Baer believed that the four great 

 animal groups, Vertebrata, Arthropoda, Mollusca, and 

 Eadiata develop from the egg by processes somewhat 

 different from each other. Nevertheless all recent 

 progress tends to show a fundamental uniformity. 



The ovum may be regarded as a simple cell which 

 undergoes division and repeated subdivision, so that 

 the segmentation may appear as 2, 4, 8, 1G, or other 

 such portions ; each of which in turn becomes a sepa- 

 rate cell.* These cells differentiate, spread themselves 

 into layers, form membranes, and finally out of these 

 are elaborated the special organs and tissues of the 

 animal body. 



* See PI. II, figs. 1 to 5, copied from Gegenbaur. 



