220 BIOLOGY AND ITS MAKERS 



day of microtomes, Von Bacr made use of sections to represent 

 the relationships of his four germ-layers. At C and D is 

 represented diagrammatically the way in which these layers 

 are rolled into tubes. He showed that the central nervous 

 system arose in the form of a tube, from the outer layer ; the 

 body-wall in the form of a tube, composed of skin and muscle 

 layers; and the alimentary tube from mucous and vascular 

 layers. 



The generalization that embryos in development tend to 

 recapitulate their ancestral history is frequently attributed to 

 Von Baer, but the qualified way in which he suggests some- 

 thing of the sort will not justify one in attaching this con- 

 clusion to his work. 



Von Baer was the first to make embrvolo«^v trulv com- 

 parative, and to point out its great value in anatomy and 

 zoology. By embryological studies he recognized four types 

 of organization — as Cuvier had done from the standpoint of 

 comparative anatomy. But, since these types of organiza- 

 tion have been greatly changed and subdivided, the impor- 

 tance of the distinction has faded away. As a distinct break, 

 however, with the old idea of a linear scale of being it was 

 of moment. 



Among his especially noteworthy discoveries may be 

 mentioned that of the egg of mammals (1827), and the noto- 

 chord as occurring in all vertebrate animals. His discovery 

 of the mammalian egg had been preceded by Purkinje's 

 observations upon the germinative spot in the bird's egg 



(1825). 



Von Baer's Rank. — Von Bacr has come to be dignified 

 with the title of the "father of modern embryology." No 

 man could have done more in his period, and it is owing to 

 his superb intellect, and to his talents as an observer, that he 

 accomplished what he did. As Minot says: "He worked 

 out, almost as fully as was possible at this time, the genesis 



