626 REPRODUCTION. 



In the new-laid egg, the blastoderm is already composed of two 

 layers. The external layer, at this time the more completely formed 

 of the two, consists in its central portions of closely packed cylindrical 

 cells, in several superimposed ranges ; and toward its outer borders of 

 a single range of flattened cells, placed edge to edge. The internal 

 layer consists of rounded cells, more coarsely granular than those of 

 the external layer, and less closely consolidated into a continuous mass. 

 This is the condition of the blastoderm in the fecundated fowl's egg, at 

 the time of its discharge from the generative passage. 



Incubation of the Egg and Formation of the Embryo. 



When the fecundated egg is discharged from the generative passage 

 and allowed to cool, the process of development is suspended at the 

 point above described. The formative changes in the blastoderm 

 require for their accomplishment a warmth nearly equal to that of the 

 fowl's body, namely, about 40 C. ; and the egg, if kept at lower tem- 

 peratures, may remain inactive for a considerable time without losing 

 its vitalit} r . When the necessary warmth is again supplied, by natural 

 or artificial incubation, development recommences and goes on to the 

 formation of the embryonic tissues. 



Extension of the Blastoderm. The first modification in the egg dur- 

 ing incubation is the increase in size of the blastoderm. This mem- 

 brane has already become larger than the cicatricula from which it was 

 produced ; for while the average diameter of the cicatricula before seg- 

 mentation is about three millimetres, the blastoderm in the new-laid 

 egg measures from three and a half to four millimetres (Kolliker). 

 But when incubation commences, it expands so rapidly that in twenty- 

 four hours it is 11 or 12 millimetres in diameter, and by the end of the 

 second day it reaches twice that size. By the continued expansion of 

 its borders it covers more and more of the spherical yolk, passing after 

 a time the equatorial line and approaching- its opposite pole. At the 

 end of the fourth day there is only a small space which it has not yet 

 covered, and by the sixth day it has completely enveloped the yolk in 

 a sac-like, membranous extension. The nutritive vitellus is thus finally 

 enclosed by the expanding blastoderm. 



Area Pellucida and Primitive Trace. The next most striking feat- 

 ure of the incubated egg is the appearance, at the central part of the 

 blastoderm, of the circular spot, known as the "area pellucida." It is 

 so called from its transparent appearance, due to the uniform structure 

 and close approximation of the cells of the external blastodermic layer 

 in this situation. The area pellucida occupies about one-half the extent 

 of the whole blastoderm, which is at this time from four to five milli- 

 metres in diameter. It is surrounded by the remaining non-transparent 

 portion of the blastoderm, the "area opaca," the opacity of which is 

 due to the fact that its internal blastodermic layer, formed of large, 

 loosely packed and rounded cells, is two or three times as thick as the 

 external layer. In the area pellucida, on the contrary, the principal 



