HISTORY OF THE GERM-LAYER THEORY. 155 



merit, thus forming the first foundation of the body. Hereupon it 

 produces new folds, which, in contradistinction to the first, give shape 

 to the abdominal and thoracic cavities, together with their contents. 

 And for the third time it sends out folds to envelop in suitable 

 membranes the foatus, which is formed out of it and by means of it. 

 Therefore it need not surprise any one if, in the course of our 

 narration, so much is said about folds and envelopes." And in 

 order to avoid misunderstandings he adds in another place the 

 important statement that "wherever anything is said about the 

 folds of the skin, one is not to imagine a lifeless membrane, whose 

 mechanically produced folds would necessarily spread themselves over 

 the whole surface, without allowing themselves to be limited to a 

 definite space. The folds which cause the metamorphosis of the skin 

 are rather themselves of organic origin, and are produced at the 

 appropriate place, either through increase in the size of the spherules 

 already present there, or through an accession of new spherules, 

 without the remaining part of the blastoderm being thereby altered." 



PANDER'S successors have expressed themselves concerning the 

 mechanism of foldings much less clearly ; the most of them, indeed, 

 not at all. The whole doctrine was in fact condemned by RUDOLPH 

 WAGNER as positively erroneous. " It will occur to no one," he says 

 in his " Lehrbuch der Physiologie," " to imagine the three germ- 

 layers to be like the leaves of a book. No one will entertain the 

 mechanical conception that the embryo arose by a folding process of 

 these three layers." 



After PANDER. LOTZE was the next to be occupied with the 

 " Mechanik der Gestaltbildung," as has been pointed out by RAUBER 

 in a meritorious history of this topic. He designates "unequal 

 growth " or " unequal vegetation " as the cause of the changes of 

 place, which in part only appear to be shif tings, out-pocketings, 

 irivaginations, or extensions, but in part are actually such, being 

 brought about in this way by mechanical traction and pressure. 



In very recent times His has prosecuted the study of embryology 

 from the mechanico-physiological standpoint more intensely than all 

 his predecessors, and has also particularly emphasised the signifi- 

 cance of the process of folding for the formation of the body. The 

 two principal writings of His in this connection are: "Unter- 

 suchungen liber die erste Anlage des Wirbelthierleibes " (1868), 

 and " Unsere Korperform und das physiologische Problem ihrer 

 Entstehung " (1874). While I refer for details to the original papers, 

 I remark that, notwithstanding manifold agreements, I cannot 



