348 THE CLASSICAL TRADITION 



" poecilogeny " was that of the shrimp J^i/innoiietcs i 



the fresh-water form of which develops in an entirely 



different way from the salt-water form. 



Experimental workers indeed were inclined to rule the 

 law out of account, to disregard completely the historical 

 clement in development, and this was perhaps the chief 

 weakness of the neo-vitalist systems which took their origin 

 in this experimental work. 



From the side also of descriptive morphology the bio- 

 genetic law underwent a critical revision. It was studied as 

 a fact of embryology and without phylogenetic bias by men 

 like Oppel, Keibel, Mehnert, O. Hertwig and Vialleton, 1 and 

 they arrived at a critical estimate of it very similar to that 

 of von Baer. 



Theoretical objections to the biogenetic law had been 

 raised from time to time by many embryologists, but the 

 positive testing of it by the comparison of embryos in respect 

 of the degree of development of their different organs starts 

 with OppePs work of iSgi. 2 He studied a large number of 

 embryos of different species at different stages of their 

 development, and determined the relative time of appearance 

 of the principal organs and their relative size. His results 

 are summarised in tabular form and have reference to all the 

 more important organs. He was led to ascribe a certain 

 validity to the biogenetic law, but he drew particular attention 

 to the very considerable anomalies inthe time of appearance 

 which are shown by many organs, anomalies which had been 

 classed by Haeckel under the name of heterochronies. 



Oppel's main conclusions were as follows: "There are 

 found in the developmental stages of different Vertebrates 

 ' similar ontogenetic series,' that is to say, Vertebrates show 

 at definite stages similarities with one another in the degree 

 of development of the different organs. Early stages 

 resemble one another, so also do later stages ; equivalent 

 stages of closely allied species resemble one another, and 

 older stages of lower animals resemble younger stages of 



1 Un problhne dc Devolution. I.u loi Ino^'ni'litjue fondamnitiilc. 

 Paris and Montprllirr, 1908. 



: VergUichvng des Entovickelungsgrades lierOrganc 

 Entwickelungsxeiten l>ei IVh-hdtiercn, Jena, 



