628 REPRODUCTION. 



todermic layer, or Ectoderm, which produces the cerebro-spinal axis 

 and the tegumentary epidermis; 2d, the internal layer, or Entoderm, 

 producing the intestinal and glandular epithelium ; and 3d, the inter- 



FIG. 183. 



.t Md 



TRANSVERSE SECTION OF BLASTODERM OP FOWL'S EGG, at the situation of the primitive trace 

 and primitive furrow. Ed. Ectoderm. Md, Mesoderm. Eni, Entoderm. (Kolliker.) 



mediate layer, or Mesoderm, from which the great mass of the mus- 

 cular system, the blood and circulatory apparatus, and the vascular 

 tissues in general are subsequently developed. 



Folds of the Blastoderm. The form of the embryo and its different 

 parts is sketched out, in all cases, by a series of folds, which show 

 themselves at various points in the blastoderm. This membrane pre- 

 sents at first a flat surface ; or, if it have a certain degree of convexity, 

 corresponding with that of the yolk upon which it lies, this convexity 

 is perfectly uniform, and too slightly pronounced to be appreciable 

 within the limits of the blastoderm. But as soon as development 

 begins to make definite progress, this uniformity of surface is broken 

 by the appearance of transverse and longitudinal folds, forming lines 

 of separation between different parts of the blastoderm. Such a fold, 

 running in a curvilinear direction from side to side, marks the position 

 of the head of the embryo, and is called the " head-fold." Its free 

 border, projecting above the neighboring portion of the blastoderm, 

 becomes the head, which, as well as the neck, is curved forward and 

 downward, in the subsequent stages of growth, with the deepening of 

 the fold which first gave it origin as a distinct part. A similar fold at 

 the posterior portion of the area pellucida, marks off the hinder ex- 

 tremity of the embryo, and is called the " tail-fold." Longitudinal 

 folds, formed in the same manner on each side, fix the lateral limits of 

 the body of the embryo. 



By this means a certain portion of the blastoderm becomes marked 

 off from the rest. The part included within the transverse and longi- 

 tudinal folds is the body of the embryo ; while that remaining outside 

 these limits becomes developed into accessory organs, playing an im- 

 portant but secondary part in the history of development. Similar 

 folds of the blastoderm also make their appearance within the body 

 of the embryo, and are the principal means of formation for its differ- 

 ent organs. A pair of longitudinal ridges, adjacent to the median line, 



