thirteenth days grows longer as the fingers grow. (Tigure 18, 

 14 - 16). The remains of this web then shortens (Figure 18, 

 18) . At this same time appear rudiments of the scales which 

 will later cover the legs; the scales at the beginning are 

 completely transparent. On the seventeenth day all the scales 

 are finally formed, but so thinly that one can see all the 

 vessels (Tigure 18, 17] through them. The posterior surface 

 of the fingers is not covered by scales, but by something of 

 a horny wart (Figure 18, 12(3), 18(4)). The claws are first 

 apparent as of the eleventh or twelfth day. In this period 

 they are straight and soft (Figure 18, 16(2)); later they 

 gradually become curved, and on the sixteenth day they acquire 

 their final form (Figure 18, 13). Only later do they enlarge 

 and get harder (Figure 18, 18(1)). 



In this section, Tredern suggested new facts and signifi- 

 cant corrections to the descriptions given by previous 

 erabryologists. Tredern 's poor optical facilities (he frequently 

 points out that he made observations with his naked eye or 

 with a lens) did not allow him to investigate the early stages 

 of development or the more delicate structure of embryonic 

 parts. In this respect he could not go beyond Wolff, whose 

 name Tredern always mentioned with deep respect. Generally it 

 is thought that Wolff's outstanding work on intestinal 

 development was overlooked by his contemporaries and only 

 rediscovered by Meckel. Even Baer, who knew Tredem's disserta- 

 tion well, argued so. But it must be acknowledged that Tredern 

 and not Meckel was the first to give credit to the classical 

 work of that Russian academician. Tredern also considered 

 mainly the later stages of chick development which had not 

 been investigated prior to him. These related to the fate of 

 the yolk intestinal duct, the development of the facial parts 

 of the skull and beak, and the development of the extremities. 

 Baer rightly noted that Tredern f s observations were repeated by 

 other embryologists many years later. 



Against the background of neglect for Tredem's work, his 

 strict and serious study of the facts was readily overwhelmed 

 by the attractive but unsubstantial fantasy of Naturphilosophie. 

 Not without reason did the young Baer, feeling a decided 

 hostility towards the influence of Naturphilosophie, become 

 immediately fascinated by the small notebook of the unknown 

 embryo logi st . 



225 



