1010 



EMBRYOLOGY. 



unnecessary here to enter into a consideration of the very complex details of its 

 structure and mode of formation. 



The above description shows the fundamental identity between the holoblastic 



DIAGRAMMATIC SECTION OF A FOWL'S UNHATOHED EGG. 



I., Blastoderm; w.y., white vitellus, consisting of a pyrit'orm central mass and a certain number of 

 concentric layers ; y., yellow vitellus, in alternate layers with the preceding ; v.t., vitelline 

 membrane; x., layer of albuminous fluid surrounding the vitellus; w., consistent albumen ; ch.L, 

 chalazae; a.ch., air-chamber; i.s.m., inner layer of the testaceous or shell membrane; s.m., 

 external layer of ditto ; s., shell. 



Fig. 547. 



and meroblastic ova ; they only differ in the enormous predominance of the 

 nutritive vitellus in the latter. This predominance has the mechanical effect of 



localizing the phenomena of segmentation to 

 one pole of the egg — that in which is the plastic 

 vitellus ; and we have seen that this is com- 

 pleted at the moment of laying. The blastoderm 

 that issues from this centre of proliferation 

 necessarily assumes the shape of a disc — the 

 germinal disc ; then of a spherical cap lying on 

 the yolk by its concavity, and tending to em- 

 brace it by the progressive extension of its 

 circular border. This investment is only com- 

 pleted towards the sixth day of incubation, and 

 the blastoderm is therefore late in assuming 

 the vesicular form. Notwithstanding differences 

 of the same kind, arising from another manner 

 of proceeding than that observed in Mammals, 

 the successive formation of the germinal area, 

 and its doubling into an opaque and a trans- 

 parent zone, take place. 



The appearance of the embryo is manifested 

 by a temporary formation known as the primitive trace — embryonal rudiment, or 

 axile layer. The primitive trace is marked on the blastoderm by two dark streaks 



pr.s 



AREA PELLUCIDA IN THE VERY EARLY 

 BLASTODERM OF THE CHICK, SHOWING 

 THE PRIMITIVE LINE AT ITS FIRST 

 APPEARANCE. 



pr.s, primitive trace ; ap, transparent 

 or pellucid area.; a.op, opaque area. 



