TISSUES IN GENERAL. 113 



Strieker (" Manual of Histology," American edition, 1872), in speaking of 

 the development of the fowl's germ, says : " The cells of the tinder layer 

 change their form and arrangement during the first hours of incubation. 

 They become flattened, and, when seen in transverse section, appear spindle- 

 shaped. Hence, after incubation has gone on for a few hours, we can ascer- 

 tain, beyond even the shadow of a doubt, that there are two and only two 

 layers. . . . The under layer, immediately after its separation from the 

 subdivided germ, consisted in some places of a single thickness of cells, 

 while in other places, in a transverse section, small heaps of cells could be 

 recognized projecting from the layer. . . . Peremeschko, however, has 

 made the communication that the large granular cells lying on the bottom of 

 the germ-cavity increase very considerably in numbers during the first hours of 

 incubation. Now, since with this increase in numbers there is not at the same 

 time a corresponding diminution in size, it is very natural to suppose that the 

 cells which project from the under germ-layer fall to the bottom of the cavity. 

 This supposition appears all the more probable when we recall the fact that 

 some of the elements of segmentation which are situated in the lower portion 

 of the germ, remain lying at the bottom of the cavity at the time when the 

 germ, in the production of this very cavity, separates itself from the subjacent 

 parts. . . . We are led to conjecture that the process is one of trans- 

 location ; that the granular bodies, which before lay at the bottom of the 

 cavity, have found their way to the space between the two first germ-layers." 

 Strieker, based upon Oellacher's researches, says that similar relations are 

 also found in the trout's germ. 



At present, investigators agree that the body of vertebrates is 

 at first a flat sheet, consisting of three main layers, for the desig- 

 nation of which the following names, have been proposed : Exo- 

 derma, Mesoderma, and Entoderma, or, preferably, epiblast, the 

 tipper layer ; mesoblast, the middle layer; and hypoblast, the under 

 layer. Of these, the epiblast and hypoblast are very thin, composed 

 of but one layer of plastids, whereas the mesoblast is a bulky 

 heap of plastids, all of which are interconnected and represent the 

 main mass of the future organism. As the originally flat sheet 

 of the germ becomes curved downward, so that the two lateral 

 halves are bent toward the median line, where they grow to- 

 gether, cavities are formed in the interior of the germ, which are 

 lined by the under layer and its derivatives. The horny layer 

 furnishes the external covering of the body and the lining of the 

 external glands, while the under layer provides the lining of the 

 intestinal cavity and its glandular organs. Linings of this descrip- 

 tion are called " epithelia," and it follows that the epiblast and 

 hypoblast give rise to all epithelia, viz. : the epiblast to those of 

 the skin and its epithelial formations (including the crystalline 

 lens) ; the hypoblast to those of the intestines and their glandu- 

 lar elongations and accumulations. The main bulk of the body 



