VON BAER AND RISE OF EMBRYOLOGY. 



115 



and relatively simple. Given a leaf-like rudiment, with the layers 

 held out by the yolk, as is the case in the hen's egg, and it was no easy 

 matter to conceive of how they are transformed into the nervous system, 

 the body wall, the alimentary canal and other parts, but, Von Baer 

 saw deeply and clearly that the fundamental anatomical features of 

 the body are assumed by the leaf-like rudiments being rolled into tubes. 

 Fig. 8 shows four sketches taken from the plates illustrating Von 

 Baer's work. At A is shown a stage in the formation of the embryonic 



Fig. 8. Sketches from Von Baer's Ejibryological Treatise (1828). 



envelojoe, or amnion, which surrounds the embryos of all animals above 

 the class of amphibia. At B, another figure of an ideal section, shows 

 that long before the day of microtomes, Von Baer made use of sections 

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

 is represented, diagramatically, 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 recapitu- 

 late their ancestral history is frequently attributed to Von Baer, but 

 the qualified way in which he suggests something of the sort will not 

 justify one in attaching this conclusion to his work. 



Von Baer was the first to make embryology truly comparative, and 

 to point out its great value in anatomy and zoology. By embryological 



