DEVELOPMENT OF CHICK 765 



composing the remainder of the egg. In consequence, the activity, of 

 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, 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 con- 

 trasted 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 anteriorl}', and constitute the mesoderm. 

 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 endoderm 

 cells becomes differentiated to form the notochord. At its sides the 

 sheets of mesodermic cells split into an inner or splanchnic layer, and 

 an outer or somatic layer. A little later the mesoderm divides into 

 the segmentally arranged mesodermic somites, lying at the sides of the 

 notochord, and the unsegmented lateral plate, whose outer and inner 

 walls form the corresponding boundaries of the coelom. 



At the time when the notochord has appeared internally, the 

 external ectoderm becomes differentiated to form the medullary groove, 

 which gives rise in the usual way to the medullary canal. The folds 



