EMBRYOLOGY 399 



gradually becomes cellular, is of considerable importance later, and 

 is known as the formative ring or germ wall. During the process 

 just described the central cells have become thinner and somewhat 

 transparent so forming an internal area known as the area pellucida, 

 while the marginal cells, composed in part of cells derived from its 

 inner margin, is denser and less transparent and constitutes the 

 area opaca. This, then, may be taken as the completion of the 

 blastula stage although its formation and final appearance is quite 

 unlike those of either Amphioxus or Rana owing to the enormous 

 amount of yolk that is present in the ovum at the beginning. 



Gastrulation or the differentiation of the two primary 

 germ layers now takes place, but the method by which it is accom- 

 plished has not been satisfactorily determined in the case of the fowl, 

 so that the account given here is based on the process as it occurs 

 in the Pigeon. The area pellucida extends slowly and its increase 

 is accompanied by a rearrangement of its cells, which finally become 

 single layered towards the posterior end at which point they extend 

 backwards forming a break in the germ ring. At this point a few 

 of the cells on the edge turn in underneath and constitute the fore- 

 runners of the entoderm. This process increases in rate and extent 

 and the place at which it takes place becomes a small, slightly 

 raised crescentic ridge. Since this contains both ectoderm and 

 entoderm cells, it can be regarded as the dorsal lip of the blastopore. 

 These entoderm cells spread forwards and outwards under the 

 ectoderm of the blastoderm and above the segmentation cavity in 

 irregular groups and form a continuous sheet only at the hinder end. 

 As a rule the egg is laid in this condition, i.e. with gastrulation not 

 properly completed, and laying takes place twenty-one or twenty- 

 two hours after ovulation. The blastoderm has increased slightly 

 by this time and may measure as much as 5 mm. in diameter. It 

 remains in this stage after it has been laid unless the temperature 

 is raised either by its being placed under a sitting hen or in an 

 incubator. These conditions induce incubation or a continuation 

 of development outside the body of the parent. During the first few 

 hours of incubation the formation of the entoderm is completed and 

 the diploblastic condition established. In the same period also, 

 we find that the ectoderm cells over the central region have become 

 decidedly columnar, whereas towards the periphery they remain 

 flat and so a thicker median area, the embryonal shield, is produced. 



The formation of the third layer is heralded by the appear- 

 ance of an opaque thicker band in the middle line of the hinder half 

 of the blastoderm about nine hours after the beginning of incubation. 

 This is the primitive streak and sections show that it is constituted 

 at first by an aggregation of ectodermal cells in that region, With 



