1012 



EMBRYOLOGY. 



The outline of the embryo distinctly appears as a very elongated ellipse. 

 At its inferior extremity is seen (Fig. 549) a very slender crescent, the per- 

 spective of which does not give so good an idea of its disposition as longitudinal 

 sections do. This appearance of the blastoderm seen on the surface, is due to the 

 inflection of its margins downwards and inwards. The elliptical disc represented 

 by the embryo, and which is lying flat on the yolk, gradually has its borders 

 curved downwards, and converging towards the middle on its ventral face. This 

 inflection of the embryo brings about the formation of the walls of the body 

 and its cavities. In their progressive incurvation, the borders circumscribe an 

 aperture, which contracts and becomes the umbilicus. The inflection proceeds 

 from before to behind, and is constituted by successive folds, which have been 



Fig. 550. . 



AREA PELLUCIDA OF THE BLASTO- 

 DERM OF A CHICK SOON AFTER 

 THE FORMATION OF THE PRIMI- 

 TIVE GROOVE. 



pr, Primitive trace with the primitive 

 groove ; af, amniotic fold. The 

 shaded part around the primitive 

 trace shows the extension of the 

 mesoblast. 



AREA PELLUCIDA IN A BLASTODERM OP 

 EIGHTEEN HOURS, SHOWING THE 

 MEDULLARY GROOVE. 



pr, Primitive trace; m.c, medullary groove, 

 or dorsal furrow; A, medullary fold. 



designated, from their situation, as the cephalic fold, lateral fold, and caudal 

 fold. 



Fig. .549 shows precisely the commencement of the cephalic fold — that is, 

 the first trace of that grand phenomenon which leads to the formation of the 

 great splanchnic cavities. It may be remarked that, throughout, the three layers 

 of the blastoderm engaged in forming the embryo are continued by their borders 

 with the other parts of the blastoderm. It may also be stated that the embryo 

 is only a morphological specialization of the blastodermic vesicle, and that there 

 is reason to distinguish the embryonal blastodermic layers from the extra- 

 embryonal layers. "We shall soon see the why and wherefore, by reason of the 

 continuity of the two regions of the blastoderm, of the modifications in the 

 embryonal layers bringing about correlative modifications in the extra-embryonal 

 layers. 



