196 



Mr. W. Heape. On the Germinal Layers [Dec. 22, 



sections, and posteriorly joins the anterior wall of the neurenteric 

 canal, while the hypoblast extends as a distinct layer below the 

 anterior end of the primitive streak. It appears highly probable that 

 the whole layer in the seven sections at the front end of the embryonic 

 area, is the hypoblast originally present there engaged in the act of 

 budding off mesoblast, as Balfonr believes to be the case with regard to 

 a similarly situated portion of the hypoblast in the chick (" Compara- 

 tive Embryology," vol. ii, p. 129 et seq.). In the three following sections 

 where distinct layers of mesoblast and hypoblast are found, the whole 

 of the mesoblast, with the exception of the cells forming the central 

 thickening in the second and third sections, has, I believe, a similar 

 origin, and may be distinguished by the form and appearance of its cells 

 from the mesoblast of the primitive streak. The mesoblast derived 

 from the hypoblast may be called hypoblastic mesoblast ; it joins the 

 mesoblast of the primitive streak as the latter grows forward, and the 

 two become indistinguishable. 



The primitive streak presents in section a similar appearance to that 

 of the embryo last described; a groove — the primitive groove — is, 

 however, present along its upper surface. The position of the future 

 neurenteric canal is indicated by a pit in the epiblast as in the specimen 

 described above. 



At a slightly later stage the embryonic area being 1*17 by *81 

 millim., the condition of the layers is much the same. A neurenteric 

 canal now perforates the whole thickness of the blastoderm at the 

 front end of the primitive groove. The upper opening of this canal, 

 which is longer than the lower opening, has the appearance of a. slit 

 with its anterior wall sloping obliquely backwards ; this wall is con- 

 tinuous with the thickening of mesoblast cells in the axial line which 

 I described in the last stage. The first traces of the amnion are 

 now visible, as a fold of the epiblast round the whole circumference 

 of the embryonic area ; at the posterior end the folds of the two sides 

 meet to form a hood, covering the hinder part of the area, but 

 anteriorly I have been unable to determine the extent of their 

 growth. 



In the surface view of an embryonic area measuring '97 by '79 

 millim. in diameter, a band of a lighter shade than the remainder is 

 to be seen in the front part of the long axis of the area, its posterior 

 end adjoining the anterior end of the primitive streak; the latter 

 occupies the hinder third of the area, and where it joins the light- 

 coloured band a pit, the upper opening of the neurenteric canal, is 

 distinctly to be seen surrounded by a dark rim. 



In transverse section the light-coloured band is seen to be caused 

 by a diminution in thickness of the epiblast plate and of the mesoblast 

 in the middle line. The epiblast of this region is bent inwards to 

 form a groove, the medullary groove ; it is wide and shallow through- 



