528 DEVELOPMENT OF THE SIMPLE TISSUES, BY S. STRICKER. 



this rudiment consists already in part of the characteristic cells 

 of the subsequent mesoblast, though in part also of the cha- 

 racteristic coarsely granular large elements (M, fig. 404); the 

 upper lamina appears to be tolerably sharply defined from the 

 above-mentioned layer. The central portion of the meso- 

 blast is thus developed at an earlier period than the rest. 

 Preparations in which the central part was already developed, 

 exhibited on both sides of it, in the space between the epiblast 

 and endoblast, as far as to the periphery, and somewhat be- 

 yond it, new-formed characteristic cells of the mesoblast, in the 

 form of thin layers or small heaps, and sometimes between these, 

 again, the large coarsely granular elements in which the 

 transitional forms from the latter to the heaps of cells may be 

 distinguished. From this observation it must be admitted that 

 the mesoblast is developed from the large coarsely granular 

 elements. 



Fig. 404. 



Fig. 404. Section of the germ of the Fowl on the first day of incu- 

 bation. 



We thus see that certain morphological elements gradually 

 diminish in number at one point (floor of the germ cavity), 

 where they were previously abundant, whilst quite similar 

 elements make their appearance in an adjoining cavity (between 

 the 'epiblast and endoblast), and then augment in number, 

 and become converted into collections of smaller cells. 



It therefore seems probable that a migration or displacement 

 occurs, by means of which the granulated structures that pre- 

 viously lay upon the floor of the cavity are introduced between 

 the two first germ laminae. 



At about the twenty-third hour of incubation all three of 

 the germ laminge are completely developed. The cells of each 

 lamina possess such characteristic features, that they may easily 



