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. 
Il. Tue PRIMITIVE STREAK 
Total Views. The primitive streak appears early on the first 
day of incubation as an elongated slightly opaque band occupying 
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