5i6 BIRDS. 



disc of formative protoplasm, in the midst of which lies the nucleus or 

 germinal vesicle. 



In the upper portion of the oviduct, the ovum is fertilised, and as it 

 passes down the duct it becomes surrounded, first by white of egg or 

 albumen, and afterwards by shell-membranes and a shell. 



The fully-formed egg is surrounded by a porous shell of carbonate of 

 lime. Within this is a double shell-membrane, the two layers of which 

 are separated at the broad end of the egg to form an air-chamber. This 

 chamber grows larger as development proceeds, and is of some import- 

 ance, in connection with respiration, as an intermediate region between 

 the embryo and the external medium. Within the inner shell-membrane 

 lies the white of egg, consisting of alternate denser and more fluid layers 

 of albumen. It is evident that shell and shell-membranes and albumen 

 are extrinsic additions to the ovum proper, which, consisting mostly of 

 yellow yolk, lies surrounded by a vitelline membrane in the midst of the 

 albumen. From the vitelline membrane, membranous shreds called 

 chalazEe extend in the albumen towards either end of the egg. The yolk 

 is not homogeneous, but includes, besides the predominant yellow yolk, 

 what is called white yolk. This consists of a central flask-shaped mass, 

 and of several thin layers concentrically arranged in the substance 

 of the yellow yolk. The stalk of the flask-shaped mass expands on 

 the surface of the yolk as a funnel-shaped disc, overlying which is the 

 area of formative protoplasm, which has already begun to segment. 



As the ovum is heavily laden with yolk, and as the formative proto- 

 plasm lies at one pole, the segmentation must be meroblastic and 

 discoidal. It begins in the lower part of the oviduct, and is exceptional 

 in being somewhat asymmetrical. By the time the egg is laid, the 

 segmented area or blastoderm has begun to be differentiated, show- 

 ing an uppermost single layer of epiblast, and a lower hypoblast of 

 larger cells in several layers. The hypoblast is separated by a slight 

 cavity from the yolk, in which nuclei appear, comparable to the yolk- 

 nuclei noticed in connection with the development of Elasmobranchs. 



Viewed from above, the segmented area or blastoderm appears as a 

 circular disc, the central part — known as the area pellucida — being 

 distinguishable by its greater transparency from the darker peripheral 

 ring or area opaca. 



The blastoderm extends over the yolk, chiefly by the increase of the 

 area opaca. Over most of the area pellucida, which assumes an oval 

 form, the' epiblast becomes two layers deep. The posterior region of 

 the area pellucida becomes opaque, and this opacity is soon replaced 

 by a dark median streak extending forwards. This linear streak appears- 

 to be homologous with what in simple cases would be the blastopore, or 

 aperture of the gastrula; but it is difficult to prove this without an 

 explanation more detailed than is possible in an elementary work like 

 this. The opacity of the primitive streak is due to a thickening of the 

 blastoderm by multiplication of cells, some of which must be regarded 

 as mesoblastic. 



In the hne of the primitive streak, about the middle of its course, a 

 shallow groove ajDpears — the primitive groove. From the sides of the 

 primitive streak, two wings of mesoblast extend to the limits of the area 

 pellucida. 



