LEREBOULLET 207 



tion in its theoretical part. But even in this memoir 

 Lereboullet was able to show that the balance of evidence 

 was greatly in favour of Milne-Edwards' views, and his general 

 conclusions in 1854 were that "in the presence of such 

 fundamental differences, one is obliged to give up the idea 

 of one single plan in the formation of animals; while, on the 

 contrary, the existence of diverse plans or types is clearly 

 demonstrated by all the facts " (p. 79). To fulfil the 

 Academy's requirements, Lereboullet continued his work, 

 and in 1861-63 he published a series of elaborate monographs 1 

 on the embryology of the trout, the lizard and the pond- 

 snail LyvifHza,a.nd rounded offhis work with a full discussion 2 

 of the theoretical questions involved. In this considered and 

 authoritative judgment he completely disposed of Serres' 

 theories of the unity of plan and the unity of genetic forma- 

 tion. Except in the very earliest stages of oogenesis there is 

 no real similarity between the development of a Zoophyte, a 

 Mollusc, an Articulate and a Vertebrate, but each is stamped 

 from the beginning with the characteristics of its type. The 

 lower animals are not, and cannot possibly be the permanent 

 embryos of the higher animals. " The results which I have 

 obtained," he writes, " are diametrically opposed to the theory 

 of the zoological series constituted by stages of increasing 

 perfection, a theory which tries to demonstrate in the 

 embryonic phases of the higher animals a repetition of the 

 forms which characterise the lower animals, and which has 

 led to the assertion that the latter are permanent embryos 

 of the former. The embryo of a Vertebrate shows the 

 vertebrate type from the very beginning, and retains this 

 type throughout the whole course of its development ; it 

 never is, and never can be, either a Mollusc or an Articulate " 

 (xx., p. 54). 



" We are led to establish ... as the general result of our 

 researches, the existence of several types, and, consequently, 

 of different plans, in the development of animals. These 

 different types are manifested from the very beginning of 

 embryonic life; the characters distinguishing them are there- 



1 Ann. Set. nat. (4) xvi., p. 113, 1861 ; xvii., p. 88, 1862 ; xviii., p. 5, 

 1862 ; xix., p. 5, 1863. 



2 xx., p. 5, 1863. 



