STAGES B TO G. MESOBLAST. 301 



cervical region. In those vertebrates in which the medullary 

 folds do not unite at approximately the same time throughout 

 their length, they appear usually to do so first in the region 

 of the neck. 



Mesoblast. 



The separation from the hypoblast of two lateral masses of 

 mesoblast has already been described. Till the close of stage C 

 the mesoblast retains its primitive bilateral condition unaltered. 

 Throughout the whole length of the embryo, with the exception 

 of the extreme front part, there are present two plates of rounded 

 mesoblast cells, one on each side of the medullary groove. These 

 plates are in very close contact with the hypoblast, and also 

 follow with fair accuracy the outline of the epiblast. This 

 relation of the mesoblast plates to the epiblast must not how- 

 ever be supposed to indicate that the medullary groove is due 

 to growth in the mesoblast : a view which is absolutely negatived 

 by the manner of formation of the medullary groove in the 

 head. Anteriorly the mesoblast plates thin out and completely 

 vanish. 



In stage D, the plates of mesoblast in the trunk undergo 

 important changes. The cells composing them become arranged 

 in two layers (PI. 10, fig. 3), a splanchnic layer adjoining the 

 hypoblast (sp), and a somatic layer adjoining the epiblast 1 (so). 

 Although these two layers are distinctly formed, they do not 

 become separated at this stage in the region of the trunk, and 

 in the trunk no true body-cavity is formed. 



By stage D the plates of mesoblast have ceased to be quite 

 isolated, and are connected with the lower layer cells of the 

 general blastoderm. 



Moreover the lower layer cells outside the embryo now 

 exhibit distinct traces of a separation into two layers, one con- 

 tinuous with the hypoblast, the other with the mesoblast. Both 

 layers are composed of very flattened cells, and the mesoblast 

 layer is often more than one cell deep, and sometimes exhibits 

 a mesh-like arrangement of its elements. 



1 I underestimated the distinctness of this formation in my earlier paper, loc. cit., 

 although I recognised the fact that the mesoblast cells became arranged in two 

 distinct layers. 



