GERMINAL LAYERS AND EARLY DEVELOPMENT OF MOLE. 113 



the whole breadth of the embryonic area. 1q sections of the 

 region in front of the primitive streak there is present a 

 layer of cells several rows thick immediately underlying the 

 epiblast plate. In the seven anterior sections this layer is 

 seen beneath the epiblast as a mass which cannot be resolved 

 into hypoblast and mesoblast : for about three following 

 sections^ placed immediately in front of the primitive streaky 

 the layer is clearly composed of (1) a layer of flattened hypo- 

 blast below and (2) a layer of mesoblast above. The meso- 

 blast in the axial line is thickened in the last two of these 

 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 hypo- 

 blast originally present there engaged in the act of budding 

 off mesoblast, as Balfour believes to be the case with regard 

 to a similarly situated portion of the hypoblast in the chick 

 {' Comparative Embryology,^ vol. ii, p. 129 et. sec/.). 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 hypo- 

 blastic 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 appear- 

 ance 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 mm., the condition of the layers is mucli 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 continuous with the thick- 

 ening 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 



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