124 



FORMATION OF THE LAYERS. 



single row of columnar cells (ep). The lower or hypoblast segments 

 are larger, in some cases very much larger, than those of the epiblast, 

 and are so granular that their nuclei can only with difficulty be 

 seen. They form a somewhat irregular mass, several layers deep, and 

 thicker at the periphery than at the centre: they rest 011 a bed of 

 white yolk, from which they are in parts separated by a more or less 

 developed cavity, which is probably filled with fluid yolk matter 

 about to be absorbed. In the bed of white yolk nuclei are present, 

 which are of the same character, and have the same general fate, as 



FIG. 91. SECTION OF A BLASTODEBM OF A FOWL'S EGG AT THE 



COMMENCEMENT OF INCUBATION. 



The thin epiblast ep composed of columnar cells rests on the 

 incomplete lower layer /, composed of larger and more granular 

 hypoblast cells. The lower layer is thicker in some places than in 

 others, and is especially thick at the periphery. The line below the 

 under layer marks the upper surface of the white yolk. The larger 

 so-called formative cells are seen at b, lying on the white yolk. The 

 figure does not take in quite the whole breadth of the blastoderm ; 

 but the reader must understand that both to the right hand and to 

 the left ep is continued farther than 7, so that at the extreme edge 

 it rests directly on the white yolk. 



those in Elasmobranchii. They are generally more 

 numerous in the neighbourhood of the thickened peri- 

 phery of the blastoderm than elsewhere. Peculiar 

 large spherical bodies are to be found amongst the 

 lower layer cells, which superficially resemble the 

 larger cells around them, and have been called forma- 

 tive cells [vide Foster and Balfour (No. 126)]. Their 

 real nature is still very doubtful, and though some are 

 no doubt true cells, others are perhaps only nutritive 

 masses of yolk. In a surface view the blastoderm, as 

 the segmented germinal disc may now be called, ap- 

 pears as a circular disc; the central part of which is 

 distinguished from the peripheral by its greater trans- 

 parency, and forms what is known in the later stages 

 as the area pellucida. The narrow darker ring of 

 blastoderm, outside the area pellucida, is the com- 

 mencing area opaca. 



As a result of incubation the blastoderm under- 

 goes a series of changes, which end in the definite 

 \ formation of three germinal layers, and in the esta- 



-=> blishment of the chief systems of organs of the embryo. 

 The more important of these changes are accomplished 

 ^ in the case of the common Fowl during the first day 

 and the early part of the second day of incubation. 



There is hardly any question in development which Las 

 been the subject of so much controversy as the mode of 

 formation of the germinal layers in the common Fowl. The 

 differences in the views of authors have been caused to a 



