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On the Germinal Layers, fyc. of the Mole. [Dec. 22, 



continually thickening towards its hinder extremity, terminates by 

 fusing with the anterior wall of the neurenteric canal. The latter 

 structure is now open above on the floor of the posterior end of the 

 medullary groove, and extends downwards into the cells beneath, 

 though it no longer perforates the hypoblast, which is, however, some- 

 what involuted, and iudistinguishably fused with the mesoblast in the 

 median line. 



Briefly to recapitulate, I have attempted to show :— 



(1.) The epiblast of the blastodermic vesicle owes its origin as well 

 to the inner mass of segmentation spheres as to the outer layer of 

 segments. It appears to originate in two ways : 



(a.) In an early stage of development (in the mole) probably by 

 the cells of the inner mass being directly transformed into part of the 

 wall of the blastodermic vesicle. 



(5.) In a later stage (mole and rabbit) by the transformation of 

 the rounded cells of the inner mass into a plate of columnar cells, 

 which joins the part of the outer layer lying immediately above it to 

 form the epiblast plate of the embryonic area. 



(2.) The mesoblast in the mole is formed in two portions : — 



(a.) A larger portion which has its origin in the primitive streak. 



(6.) A smaller portion which is derived from the hypoblast situated 

 in front of the primitive streak. 



I have been unable to distinguish where the latter, or hypoblastic 

 mesoblast, comes into contact with the mesoblast of the primitive 

 streak, and what part these respective layers take in the future 

 development of the embryo. 



(3.) A neurenteric canal is present in the mole similar to that 

 formed in other types of Yertebrata, first appearing as a pit at the 

 anterior end of the primitive streak, while in later stages it perforates 

 the floor of the hinder end of the medullary groove. 



I may here add that I have also found in a seven days' rabbit embryo 

 a rudimentary neurenteric canal in the form of a shallow pit in the 

 epiblast at the front end of the primitive streak. 



(4.) The notochord is formed of an axial strip of cells, which un- 

 derlies the epiblast of the medullary groove, and which either never 

 become divided into mesoblast and hypoblast, or in which such a 

 division, if it does take place (as appears not impossible), is very soon 

 lost. This strip of cells is originally continuous laterally with both 

 mesoblast and hypoblast, but as the la.teral mesoblast becomes con- 

 verted into definite vertebral plates the connexion is lost. 



There can, I believe, be no doubt of the connexion of the lateral 

 hypoblast and mesoblast with the notochordal cells in the mole ; in 

 the rabbit I am inclined to believe that a similar connexion is present, 

 but my evidence on this point is not yet conclusive. 



