684 BIRDS. 
the protoplasm is unable to overcome the inertia of the yolk, and 
segmentation is meroblastic and discoidal (cf. Elasmobranchs), 
In the protoplasm of the egg horizontal and vertical furrows appear 
in rapid succession, The result, as exhibited by vertical sections, is 
to produce an upper epithelial layer of cells, separated by a small 
space from larger, more irregular cells, which are still in connection 
with the yolk on which they lie. At the circular border of the 
germinal disc the two sets of cells are continuous. According to some 
authorities, this stage represents the blastula, the upper layer of cells 
corresponding to the cells of the animal pole in the frog, the lower 
with the enormous mass of yolk on which they lie to the cells of the 
vegetative pole, the space to the segmentation cavity. 
At the next stage there appears a crescent-shaped groove. In this 
region there is an ingrowth of cells, which probably represents a 
modified process of gastrulation, and results in the obliteration of the 
segmentation cavity, and the formation of a ‘‘ sub-germinal” cavity or 
archenteron. The floor of the sub-germinal cavity is formed by the yolk, 
in which, by a process of supplementary cleavage, yollk-nuclei appear. 
This condition is that attained when the egg is laid. On surface view 
we see a central ill-defined ‘“‘pellucid area.” This, which becomes 
much more distinct during the early hours of incubation, is the area of 
the blastoderm which overlies the sub-germinal cavity, and is contrasted 
with the surrounding ‘‘ opaque area,” which lies directly on the yolk. 
At the posterior region of the opaque area, as alveady noted, there is 
the crescentic groove, where the outer and inner layers are continuous. 
After the commencement of incubation, the blastoderm spreads 
rapidly over the yolk, chiefly by the extension of the area opaca; the 
area pellucida meanwhile elongates and becomes oval. 
Another important change which also occurs in the early hours of 
incubation is the conversion of the transverse crescentic groove into the 
longitudinal primitive streak. The precise meaning of this change is 
difficult and uncertain, but there seems no doubt that the primitive 
streak represents the anterior lip of the blastopore of the frog. It runs 
down the centre of the area pellucida and is marked by a central furrow, 
the primitive groove. At its sides two wings of cells are obvious ; these 
soon spread out laterally and anteriorly, and constitute the mesoblast. 
All three layers of the embryo are connected at the sides of the 
primitive streak, as at the margin of the blastopore in the frog. 
In the region in front of the primitive streak, a row of hypoblast 
cells becomes differentiated to form the notochord. At its sides the 
sheets of mesoblastic cells split into an inner or splanchnic layer, and an 
outer or somatic layer. A little later the mesoblast divides into the 
segmentally arranged mesoblastic somites, lying at the sides of the noto- 
chord, and the unsegmented lateral plate, whose outer and inner walls 
form the corresponding boundaries of the ccelom. 
At the time when the notochord has appeared internally, the external 
epiblast becomes differentiated to form the medullary groove, which 
gives rise in the usual way to the medullary canal. The folds at first 
diverge posteriorly on either side of the primitive streak, but as the 
union travels backwards, this is included in the medullary canal, and so 
disappears. 
