4 ON EMBRYOLOGY. 



had under the warmth of summer already made considerable 

 progress in development. 



The man who first logically worked out a theory of 

 evolution and became its most distinguished and zealous 

 advocate was Haller (Sur la Formation du Coeur dans le 

 Poulet, 1758, and Elementa Physiologies, Liber xxix. 1766). 



This great anatomist insisted that the embryo existed 

 even in the unincubated egg though in a rudimentary form, 

 and indeed invisible. He supposed that it was a vermiform 

 structure composed of all the essential parts of a full-grown 

 animal in an undeveloped state, and that the effect of incu- 

 bation was to educe or evolve these undeveloped organs into 

 an adult condition. The same views were urged with cha- 

 racteristic extravagance by Bonnet (Considerations sur les 

 corps organises, 1762). 



This doctrine of evolution or prsedilineation, as it was 

 called at the time, was doomed to be overthrown even in 

 Haller's own day. 



In an inaugural dissertation entitled Theoria Generationis, 

 published 1759, Gasper Frederick Wolff laid the foundations 

 of not only modern Embryology, but modern Histology. He 

 shewed that the cicatricula of the unincubated hen's egg con- 

 sisted of a congeries of particles (such as we now call cells) all 

 alike, or divisible into groups only, and that anything like 

 distinct rudiments of an embryo were wholly absent. Out of 

 these particles the embryo was built up by means of a series of 

 successive changes (several of which he described in detail, 

 especially in his work on the Formation of the Alimentary 

 ( anal, 1768), part being added to part, and parts once formed 

 b«-ing modified into fresh parts. Thus the old imperfect 

 theory of evolution was supplanted by a view, which, under 

 the term of epigenesis, was in reality a more complete and 

 truer theory of evolution. Wolff also shewed that all the 

 parts as well of plants as of animals could be conceived of 



