146 EMBRYOLOGY. 



The very earliest establishment of the germ-layer theory is asso- 

 ciated with the most celebrated names in the field of embryology : 

 CASPAR FRIEDRICH WOLFF, PANDER, and CARL ERNST VON BAER. 



CASPAR FRIEDRICH WOLFF, the discoverer of the metamorphosis of 

 plants, who, even before GOETTE, had clearly and distinctly stated 

 that the various organs of the plant, as, for example, the separate 

 parts of the flower, have been developed by various modifications of 

 leaf-like fundaments, also established the metamorphosis of animals, 

 for which he endeavoured to found a similar law of development. 



He showed in his important work on the formation of the 

 intestinal canal of the Chick, that it originally appeared in the egg 

 as a leaf-like structure, and that this afterwards became folded into 

 a groove, and finally converted into a tube. 



He conjectured that the remaining systems of organs might arise 

 in a similar way, and appended to the account of the development of 

 the intestinal canal the significant assertion : " It appears as though 

 at different periods, and many times in succession, various systems 

 might become formed after one and the same type, and as if they 

 might be on that account similar to one another, even though they 

 are in reality different. The system which is first produced, which 

 is first to take on a specific form, is the nervous system. When 

 this is concluded, then the fleshy mass, which really makes up the 

 embryo, is formed after the same type ; then appears a third, the 

 vascular system, which certainly ... is not so unlike the first ones 

 that the form described as common to all systems could not be easily 

 recognised in it. After this follows the fourth, the intestinal canal, 

 which, again, is formed after the same type, and appears as a com- 

 pleted independent whole, similar to the first three." 



WOLFF'S article, written in Latin, made _no impression on his 

 contemporaries ; it had to be rescued from oblivion by MECKEL, 

 who published a German translation of it in 1812. It was probably 

 by means of this translation that the attention of PANDER was 

 directed to WOLFF. PANDER, under the stimulus and direction of 

 his celebrated teacher, DOLLINGER, further developed the doctrine, 

 the germ of which was contained in WOLFF'S paper. 



In his publication, " Beitrage zur Entwicklung des Hiihnchens 

 im Ei," issued in the year 1817, PANDER distinguished in the blasto- 

 derm, as early as the twelfth hour of incubation, two thin separable 

 lamellte as the serous layer and the mucous layer, and main- 

 tained that subsequently a third, the vascular layer, was developed 

 between them. " Whatever noteworthy may subsequently occur," 



