CHAPTER X 



THE EMBRYOLOGICAL CRITERION 



PANDER'S work of 1817 was the forerunner of an embryo- 

 logical period in which men's hopes and interest centred 

 round the study of development. " With bewilderment we 

 saw ourselves transported to the strange soil of a new world," 

 wrote Pander, and many shared his hopeful enthusiasm. 

 K. E. von Baer's Entwickelungsgeschichte was by far the 

 greatest product of this time, but it stands in a measure 

 apart ; we have in this chapter to consider the lesser men 

 who were Baer's contemporaries, friends, followers or 

 critics. 



It was largely a German science, this new embryology, 

 and its leaders were all personally acquainted. Pander, 

 von Baer and Rathke were on friendly terms with one 

 another ; von Baer dedicated his master-work to Pander ; 

 Rathke dedicated the second volume of his Abhandlnngen to 

 von Baer. Interest in the new science was, however, not 

 confined to Germany. In Italy, Rusconi commenced in 

 1817 his pioneer researches on the development of the 

 Amphibia with a Descrisione anatomica dcgli organi della 

 circolazione delle larvc dellc Salamandre aqnaticJie (Pavia), in 

 which he traced the metamorphoses of the aortic arches. 

 This was followed in 1822 by his Amours des Salamandres 

 aquatiques (Milan), and in 1826 by his memoir Dn developpe- 

 ment de la grenouille (Milan). In this last paper he described 

 how the dark upper hemisphere of the frog's egg grows 

 down over the lower white hemisphere and leaves free 

 only the yolk plug ; he observed the segmentation cavity 

 and the archenteron, but thought that the former became 



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