K. E. VON BAER. — PHILOSOPHICAL FRAGMENTS. 209 



If it be true that all the larger and smaller groups of animals 

 depend upon a double relation, that of higher or lower develop- 

 ment, and that of the variation of the archetypes into differences 

 of smaller degree, and these again further, then the conception 

 of a uniserial progressive development of the whole animal 

 kingdom is incorrect. 



§ 4. Application of this View to the History of Individual 

 Development, 



Let us now apply the survey of the permanent relations of 

 form among the various perfect animals to the history of the 

 development of the individual ! 



Before all things it is clear, that the conditions which we have 

 termed the higher and lower development of the animal, coincide 

 perfectly with that histological and morphological differentiation 

 which gradually arises in the course of the development of the 

 individual (comp. Schol. III. c. d.). In this respect the agree- 

 ment is striking. The fundamental mass of which the embryo 

 consists agrees with the mass of the body of the simplest ani- 

 mals. In both the form is but little defined; the parts are less 

 contrasted, and the histological differentiation remains even 

 behind the morphological. If we cast a glance over the lower 

 animals, remark in some more internal development than in 

 others, and then arrange them in a series according to this deve- 

 lopment, or conceive them to be developed out of one another ; 

 it necessarily follows that we should trace an agreement in the 

 fact of this very progressive differentiation between the one 

 actual historical succession and the other imagined genetic 

 series ; and in this manner a multitude of coincidences may be 

 demonstrated between the embryo of the higher and the per- 

 manent forms of the lower animals. 



It by no means follows from this, however, that every embryo 

 of a higher animal gradually passes through the forms of the 

 lower animals. On the contrary, the type of every animal ap- 

 pears to be fixed in the embryo from the very first, and to regu- 

 late the whole course of development. 



All that we have stated with regard to the development of the 

 Chick is only a long commentary upon this proposition. The 

 chorda dorsaUs is the first part which becomes differentiated^ 



SCIEN. MEM.— JVa^ Hist. Vol. I. Part HI. 14 



