WOLFF ON GERM-LAYERS. 45 



layers, or leaves, is the intestinal canal, the development of 

 which Wolff examined thoroughly, from its beginning to its 

 completion. He showed that the leaf-like rudiment first 

 forms a groove, the edges of which curve towards each other, 

 thus growing into a closed tube, and that, finally, at the 

 ends of this tube the two openings, mouth and anus, arise. 



Nor did Wolff overlook the fact that the other organic 

 systems of the body originate, in an entirely similar way 

 from leaf-shaped rudiments, which afterwards assume the 

 form of tubes. Like the intestinal canal, the nerve, muscle, 

 and vascular systems, with all the various organs belonging 

 to the last, develop from a simple layer-like or leaf-like 

 rudiment. Thus in 1768 Wolff learned the very significant 

 fact, which, half a century later, was first formulated by 

 Pander, in the fundamental " germ-layer theory." The 

 sentence in which Wolff expressed the main idea of this 

 theory is so remarkable, that I quote it. "This very 

 wonderful analogy between parts which in Nature are so 

 widely separated, an analogy which is not imaginary, but 

 is founded on the most reliable observations, is in the 

 highest degree worthy of the attention of physiologists ; 

 for it will be granted that it has a deep significance and 

 that it is most intimately comiected with the generation, 

 and with the nature of animals. The different systems 

 which compose the whole animal seem to be successively 

 formed, at different times but on one plan; and these 

 systems are therefore like one another, even though in their 

 nature they are distinct. The system which is first pro- 

 duced, which first assumes a peculiar definite form, is the 

 nerve-system. When this is completed the flesh-mass, 

 which properly speaking constitutes the embryo, is formed 



