LAW OF PARALLELISM 91 



ologic" 2 pts., 1806-7) as forming the turning-point in our 

 understanding of the mammalian ovum. He had accordingly 

 actually observed a resemblance in certain details of structure 

 between the human foetus and the lower animals ; but the 

 peculiar form which the law took in his hands was a con- 

 sequence of his hazy philosophy. He saw the relation of 

 teratological to foetal structure, for he affirmed that " mal- 

 formations are only persistent foetal conditions " (p. 492). 



The idea of comparing the embryo of higher animals 

 with the adult of lower was widely spread at this time among 

 German zoologists. We find, for example, in Tiedemann's 

 brilliant little textbook x the statement that " Every animal, 

 before reaching its full development, passes through the 

 stage of organisation of one or more classes lower in the 

 scale, or, every animal begins its metamorphosis with the 

 simplest organisation " (p. 57). 



Thus the higher animals begin life as a kind of fluid 

 animal jelly which resembles the substance of a polyp ; the 

 young mammal, like the lower Vertebrates, has only a simple 

 circulation, and, like them, lives in water (the amniotic fluid); 

 the frog is first like a worm, then develops gills and becomes 

 like a fish (p. 57). In his work on the anatomy of the brain, 2 

 Tiedemann established the homology of the optic lobes in 

 birds by comparing them with foetal corpora quadrigemina 

 in man (see Serres, Ann. Sci. nat., xii., p. 112). 



J. F. Meckel, in 1811, devoted a long essay to a detailed 

 proof of the parallelism between the embryonic states of the 

 higher animals and the permanent states of the lower 

 animals. In a previous memoir in the same collection 3 

 (i., i, 1808) he had made some comparisons of this kind in 

 dealing with the development of the human foetus ; in this 

 memoir (ii., i, 1811) he brings together all the facts which 

 seem to prove the parallelism. 



His collection of facts is a very heterogeneous one ; he 

 mingles morphological with physiological analogies, and 

 makes the most far-fetched comparisons between organs 



1 Zoologie, Landshut, i., 1808. 



2 Anatomic u. Bildungsgeschichtc des Gehirns im Fotus des Menschen, 

 Niirnberg, 1816. 



3 Beytrcige zur vergleichende Anatomic, Leipzig, i., 1808-9, ii., 1811-2. 



