660 BIRDS. 



At the next stage there appears at the future posterior end 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, yolk-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 already 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. 

 The precise origin of the constituents of this middle layer is uncertain, 

 but it is important to notice that all three layers of the embryo are con- 

 nected at the sides of the primitive streak, as at the margin of the blas- 

 topore 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 cnelom. 



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. 



During the course of the second day the embryo seems to sink 

 further into the yolk, while both anteriorly and posteriorly double folds, 

 known respectively as the head and tail folds, rise up. In the course 

 of their development the embryo becomes completely "folded off" 

 from the yolk. At a slightly later stage, side folds also appear ; all the 

 folds now consist of a double layer of somatopleure, covered externally 

 by epiblast. The folds meet above the back of the embryo and coalesce. 

 The inner layer forms the true amnion, the outer the false amnion or 

 subzonal membrane. Into the space between the amniotic folds, a 



