132 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



since the yolk entoderm and its inclosed cavity are caenogenetic features. There 

 are thus restored the original conditions in which the anlage of the chorda and meso- 

 derm (enteroccele) are situated in the dorsal wall of the gut. To what extent there 

 is any formation of gastral mesoderm from the head process is a question. In any 

 c;isc, even if the mesoderm had a peristomal origin, its continuity with the walls of 

 the canal is sufficient to indicate the interpretation of the latter as potential sources 

 of gastral mesoderm. With the disappearance of the floor of the canal there is 

 ushered in the stage of the so-called intercalation of the chorda in the entoderm. 

 This obviously takes place quite irregularly and the picture is exactly that seen 

 on such a large scale in Reptilia, but clearly marked in many other forms. This 

 stage is shown on plate 2, figure 4, and plate 3, figure 4. To be exact, this is not an 

 intercalation of the chorda in the entoderm. The notochordal plate is in connection 

 laterally not only with the entoderm, but much more extensively with the mesoderm. 

 If one suppose that there may still be mesoderm formed from the borders of the plate 

 (and there is here no evidence to the contrary) it would be possible to raise objection 

 to the use of the term "notochordal plate," since it would contain chorda and 

 gastral mesoderm. Not, however, until there is ;i definite separation of the plate 

 from the mesoderm and its continuity with the entoderm alone can one speak of an 

 intercalation of the chorda. 



The extent, if any, to which the plaque lecithoenterique (Dotterdarmplatte) 

 contributes to the formation of the wall of the future digestive tract is difficult 

 to determine and certainly not to be decided by any one stage. There are a number 

 of facts, however, which seem to point to such a participation. The marked dis- 

 proportion between the notochordal and enteric plates in the posterior, least differ- 

 entiated part of the head process and the retention of the former, practically intact 

 throughout its whole extent, indicate unmistakably that there is formed from the 

 primitive node and head process a considerable mass of material which is not ex- 

 pended in the formation of the chorda. If this material be not actually lost, then 

 it must find its way into the mesoderm or entoderm or into both. In view of the 

 large mass of material produced, much greater than that destined for the chorda, 

 and considering also its peculiar mode of development, virtually an invagination, 

 the simplest solution is to suppose that both mesoderm and entoderm are formed 

 from the side-wall and floor of the head process. If the development of gastral 

 mesoderm is small or wanting, so much more material for the entoderm. It may 

 be recalled here that the digestive tract in the embryo is very small below the pharynx 

 and no very great amount of material would be required to form its walls. The fact 

 that in certain animals the primary entoderm is concerned in the formation of the 

 epithelial wall of the gut seems to us very significant. It would seem that the 

 absence of definite evidence that the entoderm of the future embryo is not, in part 

 at least, primary entoderm, is outweighed by the above considerations and by the 

 fundamental homologies which they tend to preserve. 



With the head process and primitive streak we have not yet exhausted the 

 possibilities for the discussion of fundamental problems; there remains the question 

 of the completion plate, Bonnet's Erganzungsplatte, the protochordal plate of 



