BIRDS' EGGS 



263 



The minimum temperature for a hen's egg is 28° C. The 

 drop of formative protoplasm on the top of the yolk divides 

 and redivides and forms a disc of cells — the blastoderm. 

 This soon proceeds to spread over the yolk, but the essential 

 events take place in an oval area in the middle of the upper 

 surface. Even on the first day the nervous system begins 

 to develop, appearing as a median longitudinal groove ; and 

 soon the foundations are laid of food-canal, musculature. 



Y 



ALB 



Fig. 43. — A, The embryonic disc seen from above. Suspended in the 

 white of egg (alb.) is the ovum proper, the egg-cell dilated with yolk (v.). 

 A formative drop of protoplasm divides and redivides, so that a disc 

 of cells results (g.d.). The central part of this disc or blastoderm is thin 

 and translucent (the area pellucida) ; the marginal part is thicker and 

 less transparent (the area opaca (a.o.)). At the inner rim of the area 

 opaca, bordering the area pellucida, there is an abundant development of 

 blood-vessels — the area vasculosa. B, Much enlarged view of embryonic 

 disc or blastoderm, seen from above, showing a pavement-like area 

 of cells. 



skeleton, and tlie like. Very rapidly, though often inter- 

 mittently, the development goes on — one of the most 

 familiar and one of the least intelligible processes in the 

 world. 



Out of apparent simplicity — a drop of living matter on 

 the top of the yolk — there emerges obvious complexity. 

 We know what is taking place — the various items or factors 



