FORMATION OF THE EMBRYO IN THE FOWI/S EGG. 627 



thickness of the blastoderm is formed by the external layer ; the inter- 

 nal consisting of only a single range of cells, often incompletely con- 

 tinuous. As the blastoderm enlarges, its area pellucida encroaches on 

 the space previously occupied by the area opaca ; and the area opaca 

 expands in turn, advancing beyond the borders of the transparent por- 

 tion. The area pellucida soon assumes an oval form, placed trans- 

 versely to the long axis of the egg ; and the body of the embryo will 

 afterward occupy the same position, the wider end of the oval corre- 

 sponding to the future situation of the head, its narrower end to that 

 of the tail. 



Xot long after the formation of the area pellucida, it presents in its 

 longitudinal axis a slight linear eminence, caused by local thickening 

 and condensation of the external blastodermic layer. This is known 

 as the "primitive trace." It appears, from the tenth to the fourteenth 

 hour of incubation, as an ill-defined linear opacity, about one milli- 

 metre in length and 0.2 millimetre in width, of a straight or slightly 

 sinuous form, and somewhat eccentric in position, occupying rather 

 the posterior than the anterior portion of the area pellucida It shows, 

 along the median line on its upper surface, a shallow depression, the 

 "primitive furrow,' 1 and by the fifteenth hour of incubation it is fully 

 constituted in all its parts. 



The primitive trace, although it indicates the direction of the longi- 

 tudinal axis of the future embryo, is not an initial formation of the 

 embryonic organs, and takes no direct part in their development. It 

 is a transitory structure, which disappears soon after its production, 

 and gives place to others of more permanent significance. But it is a 

 feature of much interest, as the earliest local modification in the trans- 

 parent portion of the blastoderm. 



Formation of Three Blastodermic Layers. The blastoderm, at the 

 time of its appearance in the new-laid egg, consists, as above described, 

 of two layers of cells, namely, an external and an internal. Of these, 

 the external alone is fully constituted; the internal being less complete 

 and more or less discontinuous in the central portions of the blastoderm. 

 But a few hours after the commencement of incubation, the internal 

 blastodermic layer becomes continuous throughout, forming everywhere 

 a distinct consistent expansion. Within the limits of the area pellucida 

 its cells assume a flattened form, being thus still further distinguished 

 from those of the external layer, which in this situation are more 

 cylindrical in figure and multiply with great rapidity. Soon after- 

 ward a third blastodermic layer makes its appearance, between the 

 other two, composed of uniformly rounded cells. It is first produced 

 along the line of the primitive trace, and thence extends laterally on 

 each side, diminishing in thickness until it terminates, at some distance 

 from the median line, in a thin edge. In the region of the primitive 

 trace (Fig. 183) the blastoderm is then composed of three cellular 

 layers, which have received distinct names, and which afterward give 

 origin to all the organs of the embryo, namely : 1st, the external bias- 



