THE EARLY HISTORY QF THE MESODERM. 



79 



capsule. The capsule enters into the formation of the cranium, and when it ossifies 

 forms also the so-called bony labyrinth. 



The auditory ganglion fuses very early with the cephalad wall of the otocyst, 

 so that in sections no boundary between the ganglion and the epithelium can be 

 distinguished. The upper part develops into the vestibular, the lower into the coch- 

 lear ganglion. 



R.VIII. D.endo. 



S.C. 



R.VI1. 



Cock. 



N.VII. 



FIG 42. PIG EMBRYO OF 12 MM. SERIES 5. RECONSTRUCTION IN WAX OF THE OTOCYST, WITH PART OF THE 



BRAIN-WALL AND THE SEVENTH AND EIGHTH NERVES. BY G. C. COE and W. W. BEHLOW. VIEW FROM 



THE CAUDAD SIDE. 

 Br, Brain- wall. Cock, Cochlear anlage. D.endo, Ductus endolymphaticus. N.VII, Seventh nerve. R. 



VII, Root of seventh nerve. R. VIII, Root of eighth nerve. S.C, Region from which the semicircular 



canals are formed. X 100 diams. 



The Early History of the Mesoderm. 



Concerning the precise origin and early development of the mesoderm authori- 

 ties are by no means agreed, and in the interpretations offered there has been more 

 of hypothesis than of observation. The most accurate observations have so far 

 been made on the elasmobranchs, lizards, and chick. In these -forms the entoderm 

 (or segmenting yolk) in the neighborhood of the primitive streak produces cells 

 which take their place so as to form a layer next to the entoderm. This layer 

 gradually becomes more and more distinct until it can be definitely recognized as 

 a separate layer, the mesoderm. It is probable that a similar process goes on in 

 amphibia and in mammals, so that it is safe to say that the mesoderm probably 

 arises by this process, which we calh delamination, in all vertebrates. In its first 



