FORMATION OF THE LAYERS. 



now be called, appears 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 commencing area opaca. 

 FIG. 91. SECTION OF A BLASTODERM OF A FOWL'S EGG AT a, 



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 /'. 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 /, so that at the extreme edge 

 it rests directly on the white yolk. 



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 es- 

 tablishment 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 has 

 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 

 large extent by the difficulties of the investigation, but 

 perhaps still more by the fact that many of the observations 

 were made at a time when the methods of making sections 

 were very inferior to those of the present day. The subject 

 itself is by no means of an importance commensurate with 

 the attention it has received. The characters which belong 

 to the formation of the layers in the Sauropsida are second- 

 arily derived from those in the Ichthyopsida, and are of but 

 little importance for the general questions which concern 

 the nature and origin of the germinal layers. In the account 

 in the sequel I have avoided as much as possible discussion 

 of controverted points. My statements are founded in the 

 main on my own observations, more especially on a recent 

 investigation carried on in conjunction with my pupil, Mr 

 Deighton. It is to Kolliker (No. 135), and to Gasser (No. 127) that the most 

 important of the more recent advances in our knowledge are due. Kolliker, 







