CHAPTER IV 



FROM LAYING TO THE FORMATION OF THE FIRST 



SOMITE 



I. STRUCTURE OF THE UNINCUBATED BLASTODERM 



THERE is more or less variation in the stage of development 

 of unincubated blastoderms; in exceptional cases these variations 

 may be extreme. However, the usual condition may be described 

 very briefly as follows (see Fig. 34): Beneath the pellucid area 

 is the subgerminal cavity bounded marginally by the germ-wall. 

 The posterior part only of the pellucid area is two-layered. The 

 lower layer or gut-entoderm terminates posteriorly at the germ- 

 wall, with which, however, it is not united. It is composed of 

 spindle-shaped cells which form a coherent layer, perforated by 

 numerous small openings that appear as breaks in the layer 

 in section. In front of the gut-entoderm a few scattered cells 

 appear in the subgerminal cavity. The gut entoderm does not 

 reach the germ-wall either laterally or anteriorly, but in the 

 course of a few hours' incubation it spreads so as to unite with 

 the germ-wall around the entire margin of the pellucid area. 



The germ-wall is slightly thicker at the posterior than at the 

 anterior end, that is to say, that the nuclei extend deeper into 

 the yolk (Fig. 34). There is a broad zone of junction and beyond 

 this the margin of the blastoderm overlaps the yolk a short dis- 

 tance. The germ-wall has not yet become organized as a layer 

 separate from the yolk. 



The ectoderm is thicker in the region of the area pellucida 

 than in the area opaca; and slightly thicker in the center than 

 at the margin of the area pellucida. This thickening is in part 

 the forerunner of the medullary plate. 



II. THE PRIMITIVE STREAK 



Total Views. The primitive streak appears early on the first 

 day of incubation as an elongated slightly opaque band occupying 



