FORMATION OF BLASTODERMIC ELEMENTS. 



687 



and middle layer is yet to be perceived. According to most observers the original 

 upper layer takes no sliai-e in these changes, but remains distinct and undergoes 

 the changes which belong to its own phases of development. 



With respect to the formation of the hypolilast it would appear to be no more, 

 at least in its central part, than a differentiation or change of form occurring in 

 cells existing from an earlier period in the primitive lower layer ; while its 

 peripheral extension is probably owing to the conversion into its pavement-like 

 cells of the subjacent elements of the white yolk. But as to the manner in which 

 the mesoblast takes its origin, two distinct kinds of views exist among embryo- 

 logists. According to one of these, following the suggestion of Remak. the cells 

 of the mesoblast take their rise by a process of separation from the cells of the 

 primitive lower layer by changes which are coincident with the conversion of 

 the deepest set of those cells into the continuous lamina of hypoblast. And as 

 a modiiication of this \dew may be mentioned that of His, according to which 

 a middle layer (though not distinguished by him as such by namej arises in 

 common from the formative cells of both upper and lower primitive layers 

 through an axial plate, into which he holds they unite. 



According to the other view, which originated in the Vienna school, and has 

 received much support from a number of observers emanating from it (Strieker, 

 Waldeyer, Peremeschko, Klein, Ocllacher and Goette), the cells which form the 

 middle layer do not proceed either from the epiblast or hy^joblast in the place 

 which they ultimately occupy, but these cells arise as new products of cell forma- 

 tion below the hy^joblast, pass by migi-atory movement into the seat of the meso- 

 blast, either through the hypoblast, or, as most hold, round its peripheral margin, 

 and thence into the central part of the blastoderm, where all are agi-eed the cells 

 of the mesolDlast first come to be collected in any considerable qiiantity. Having 

 once gained this position, or. in other words, a certain portion of mesoblast 

 having been thus formed in the axial or central plate of the embryonic area, its 

 cells speedily multiply and rapiiUy extend themselves, both by thickening in the 

 centre and by spreading towards the peripliery : other mesoblastic cells continue 

 to be introduced from below at the margin of or through the hypoblast, so as to 

 complete the formation of a middle layer by the eighteenth or twentieth hour. 



FiiT. 498. 



Fig. 498. — Section of a 

 Blastoderm at right 



ANGLES TO THE LoNG 



Axis or THE Embryo, 



NEAR ITS .MIDDLE, 

 AFTER EIGHT HODlls' 



Incobation (from Fos- 

 ter utrI Balfour). 



A, epiblast formed of 

 two layers of cells ; B, 

 mesoblast thickened be- 

 low the primitive gi'oove ; 

 C, hypoblast formed of 

 one layer of flattened 

 cells ; pr, primitive 



groove ; vie, mesoblast cell ; bd, formative cells in the so-called segmentation or sub- 

 germinal cavity. (The line of separation between the epiblast and mesoblast below the 

 primitive groove is too strongly marked in the figure.) 



Among the most recent observers. Klein and Balfour favour the migratory 

 view : the latter, however, in a somewhat modified ftirm. as he has an-ived at the 

 conclusion that the mesoblast takes its origin not directly from the epiblast or 

 hypoblast, but in part from cells which are included (as the result of the first 

 segmentation) in the blastoderm between those of its upper and lower layers, and 

 partly from the larger spheres or foi-mative cells which are the product of a later 

 process of cell production occurring in the lower part of the germ, and which 

 migrate from the place of their fonnation in the germ cavity, round the margin 

 of the hypoblast into the space above it. The researches of Goette lead nearly 



