116 WALTER HEAPE. 



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 underlies the epiblast of the medullary groove, and 

 which either never becomes 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 lateral mesoblast becomes converted 

 into definite vertebral plates the connection is lost. 



There can, I believe, be no doubt of the connection of the 

 lateral hypoblast and mesoblast with the uotochordal cells 

 in the mole ; in the rabbit I am inclined to believe that a 

 similar connection is present, but my evidence on this point 

 is not yet conclusive. 



PRINIKD BT J. £. ADI.ARU. BAKl HOLUIIKW CLOSE. 



