614 BIRDS. 



oviduct. Within the enveloping albumen lies the ovum proper, with its 

 enormous mass of yolk. The yolk is not homogeneous, but consists of 

 two substances, known respectively as white and yellow yolk. The 

 white yolk forms a central flask-shaped mass, and occurs also as thin 

 concentric layers in the yellow yolk. 



On the upper surface of the yolk, in whatever position the egg be 

 held, lies the segmented blastoderm, whose exact origin we must con- 

 sider more precisely. 



In accordance with the principles of development, with which the 

 student is already familiar, yolk is to be regarded as an inert and passive 

 substance. In the hen's egg we have an increased specialisation along 

 the line indicated by the egg of the frog. Here we have a small patch 

 of formative protoplasm at one pole, and a large aggregate of yolk com- 

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

 protoplasm is unable to overcome the inertia of the yolk, and segmenta- 

 tion is meroblastic and discoidal. 



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 at the future posterior end a crescent- 

 shaped groove. In this region there is an ingrowth of cells, which pro- 

 bably 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. 



