DEVELOPMENT OF THE CHICK. 



623 



intermediate region between the embryo and the external medium. 

 Beneath the shell membranes lies the albumen, or "white of egg," 

 which is secreted by the thin-walled region of the oviduct ; in it lie two 

 spirally-twisted cords or chalazse, produced by the rotation of the egg in 

 the 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. 



The minimum temperature at which a hen's egg will develop normally 

 is 28° C. If the temperature fall below this, development stops. In 

 early stages the interruption may last for days without fatal results, 

 though always with a tendency to induce subsequent abnormalities. 

 Towards the end of incubation more than a day's cooling is usually 

 quite fatal. 



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

 held, lies the segmented blastoderm, 

 whose exact origin we must consider 

 more precisely. 



As we have seen, 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. For there is 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 (cf. 

 Elasmobranchs). 



In the protoplasm of the egg hori- 

 zontal 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 

 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. 



Fig. 274. — Diagrammatic section 

 of egg. — After Allen Thomson. 



g.v., Position of germinal vesicle; 

 a.c, air-chamber; K., yolk (alter- 

 nate layers of "yellow" and 

 "white"); ch.y chalaza. 



On surface view 



