120 KAUI. KRNST VON BAKU 



first formed as a flat plate which folded round to form a tube, 

 and in a somewhat vaguely worded passage he hinted that 

 a similar mode of origin might be found to hold good for the 

 other organ-systems. But it seems clear that Wolff had no 

 definite conception of the process of layer-formation as the 

 first and necessary step in all differentiation. This, at any 

 rate, was von Baer's opinion, who assigns to Pander the glory 

 of the discovery of the germ-layers. " You," he writes, 

 " through your clearer recognition of the splitting of the 

 germ a process which remained dark to Wolff have shed 

 a light upon all forms of development " (p. xxi.). 



We have now seen, following von Baer's exposition, 

 how development is essentially a process of differentiation, 

 a progress from the general to the special, from the 

 homogeneous to the heterogeneous ; we have analysed 

 the process into its three subordinate processes primary, 

 histological and morphological differentiation. So far we 

 have considered development in general and the laws which 

 govern it ; we have now to consider the varieties of 

 development which the animal kingdom offers in such 

 profusion, in order to discover what relations exist between 

 them. This is the problem set in the fifth Scholion. Baer 

 at once brings us face to face with the solution of the problem 

 attempted in the Meckel-Serres law. It is a generally 

 received opinion, he writes, that the higher animals repeat in 

 their development the adult stages of the lower, and this is 

 held to be the essential law governing the relation of the 

 variety of development to the variety of adult form. This 

 opinion arose when there was little real knowledge of 

 embryology ; it threw light indeed upon certain cases of 

 monstrous development, but it was pushed altogether too far. 

 It complicated itself with a belief in a historical evolution; 

 "People gradually learnt to think of the different animal 

 forms as developed one from another and seemed, in some 

 circles at least, determined to forget that this metamorphosis 

 could only be conceptual " (p. 200). At the same time the 

 theory of parallelism led men to rehabilitate the outworn 

 conception of the scale of beings, to maintain that animals 

 form one single series of increasing complexity, a scale 

 which the higher members must mount step by step in their 



