A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 133 



Hubrecht. Our knowledge of this structure is less extensive than that of the region 

 we have just been considering, and our remarks will be correspondingly brief. 

 Bonnet's term is a very fitting one, since the derivatives of the plate are the same 

 as those of the head process and directly continuous with them. For Rabl (I. c., 

 p. 239) the completion plate is simply "das vorderste Ende des in Lecithophor vor- 

 geschobenen Kopffortsatzes oder Urdarmsackchens." For Bonnet and Hubrecht 

 it is developed from the yolk entoderm independently of the head process. The 

 evidence in this particular case would seem rather to support this latter view. 

 The second view, however, is not so easily reconciled with our ideas of gastrulation 

 as the first, and we shall not carry the discussion farther at this time. Concerning 

 the future of this plate, which has been recognized in a variety of forms and given 

 a variety of names, there is much more unanimity of opinion. There arise from the 

 completion plate in the dog, according to Bonnet (/. c., p. 286) : "1. Mesoderm des 

 Vorderkopfes, 2. die Chorda des Vorderkopfes, und 3. ein pramandibulares Darm- 

 rudiment. Es bildet dieses Gebiet also thatsachlich ein Erganzungsstuck des 

 Urdarmes, indem es dieselben Derivate wie dieser aus sich hervorgehen lasst." The 

 anterior part of the chorda, which, as compared with that derived from the head 

 process is very short, remains long in connection with the entoderm. The forma- 

 tion of mesoderm is also continued here for some time. 



The significance of the apparent cavity formation in the completion plate is 

 a matter of uncertainty. It might be compared with the secondary canals which 

 sometimes appear in the chorda as it separates from the entoderm. One could 

 perhaps look upon them as attempts at the development of an archenteric cavity, 

 or they might conceivably stand in some relation to the rarely appearing head 

 cavities. As far as we can make out, the buccopharyngeal membrane would have 

 appeared close to the anterior limit of the completion plate, with the possibility 

 of the plate contributing in its formation. 



From the observations here presented and from the consideration of other human 

 embryos, one may conclude that the essential features of gastrulation in man are 

 directly comparable with the classical features of that ancient and important 

 process. Significant parallels may be drawn between early human ontogenesis and 

 that of many other representative vertebrates. The conditions in man are mani- 

 festly simpler and more primitive than in many cases which have been extensively 

 studied, these being often very specialized or aberrant forms. Hand in hand with 

 specialization and advancement there is the appearance in ontogeny of caeno- 

 genetic features which always tend to obscure the original picture. If man has 

 retained much that is primitive and generalized, then we should expect to find some 

 expression of this in his earliest development. 



CLEVELAND, OHIO, September 27, 1917. 



