864 STRUCTURE OF THE BLASTODERM. 



to the fact that there are several layers of cells in this region (fig. 662). In a 

 transverse section through the primitive streak, three layers of cells are seen. 

 They form part of the middle layer or mesoblast, and are originally derived from 

 the hypoblast. These cells fuse with those of the epiblast. The remainder of the 

 hypoblasts cells retain their stellate character.] At the same time a new layer of 



/ Fig. 662. 



Transverse section of the primitive streak of a fowl's blastoderm, cp, epiblast ; hy, hypoblast ; 

 m, mesoblast ; pv, primitive groove ; yh, yoke of germinal wall. 



cells is developed between the epiblast and hypoblast, the mesoderm or mesoblast 

 (tig. 662, I), which soou extends over the embryonal area, and into the blastoderm. 



[There has been much discus- 

 sion as to the origin of the 

 mesoblast, but in vertebrates 

 it seems to be originally de- 

 veloped from the hypoblast. 

 Fig. 663 shows a portion of 

 the hypoblast in its axial part, 

 in process of forming the noto- 

 chord, which is described as 

 mesoblastic. ] Blood - vessels 

 Transverse section of an embyro newt, a, mesenteron ; are f orme d within the meso- 

 -/'. //*/, :ixial hypoblast, forming the notochord ; be, coelom , , , ,. . ., . , 



or body-cavity; ep, epiblast f hy, digestive hypoblast ; blast > and are distributed over 

 win, somatic mesoblast ; sr>m, splanchnic mesoblast ; np, the blastoderm to form the 

 neural plate. area vasculosa. 



Medullary Groove. A longitudinal groove, the medullary groove, is formed at 

 the anterior part of the embryonal shield, but it gradually extends posteriorly, 

 embracing the anterior part of the primitive streak with its divided posterior end, 

 while the primitive streak itself gradually becomes relatively and absolutely smaller 

 and less distinct, until it disappears altogether (fig. 661, I, and II, Pr). 



The position of the embryo is indicated by the central part becoming more 

 transparent, the area pellucida, which is surrounded by a more opaque part 

 the area opaca. [The area opaca rests directly upon the white yelk in the fowl, 

 and it takes no share in the formation of the embryo, but gives rise to structures 

 which are temporary, and are connected with the nutrition of the embryo. The 

 embryo is formed in the area pellucida alone.] 



From the epiblast \neuro-epidennal layer] are developed the central nervous 

 system and epidermal tissues, including the epithelium of the sense-organs. 



From the mesoblast are formed most of the organs of the body [including the 

 vascular, muscular, and skeletal systems, and, according to some, the connective- 

 tissue. It also gives rise to the generative glands and excretory organs] . 



From the hypoblast ejnthelio-glandvlar layer [which is the secretory layer], arise 



