220 BIOLOGY AND ITS MAKERS 
day of microtomes, Von Baer 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 embryology truly com- 
parative, and to point out its great value in anatomy and 
zoélogy. 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. Asa 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 Baer 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 
