form the two halves of the cerebro-spinal axis, which afterward coalesce 

 with each other along their dorsal edges; and the formation of the 

 intestinal canal, as well as its inclosure by the abdominal walls, results 

 from the growth of lateral folds which curve downward and inward, 

 to meet on the median line below. Thus the body of the embryo, 

 consisting mainly of the thickened ectoderm and mesoderm, is at first 

 spread out, in a nearly uniform plane, over the surface of the yolk, 

 resting upon the entoderm, which represents the epithelial lining of 

 its future alimentary canal. But as the depressed folds of its lateral 

 borders penetrate more deeply below the general level, the sides of 

 the embryo shut in between them a cavity, which is afterward com- 

 pleted by the union of its edges, and thus finally embraces the ali- 

 mentary canal, with the other thoracic and abdominal organs. 



The above changes, which thus determine the configuration of the 

 embryo, result from the special activity of growth in particular parts 

 of the blastoderm. If this membrane were to grow only at its edges, 

 it would simply extend farther over the vitellus, its central portions 

 remaining as before. If it were to increase everywhere at a uniform 

 rate, it would become thicker as well as more extensive, but without 

 any special alteration of form. This is what really takes place during 

 the early production of the blastoderm, which at first expands on all 

 sides, retaining its original uniformity of surface. 



But with the commencement of incubation the blastoderm grows 

 more rapidly at particular points, and along certain lines, than else- 

 where. What may be the determining cause of such a concentration 

 of growth, it is impossible to say ; but its result is that the blastoderm, 

 enlarging with different degrees of rapidity in different regions, is 

 thrown into undulations, which indicate, by their position and size, 

 the unequal expansion of its mass. Thus, if it grow more rapidly at 

 one point than in the adjacent parts, it will form at that spot either an 

 eminence or a depression, according as it meets with less resistance 

 above or below. If a similar rapidity of increase should take place 

 along a transverse line, the consequence would be a transverse fold ; 

 and if in an antero-posterior direction, it would cause a longitudinal 

 fold. The subsequent history of embryonic development shows con- 

 tinual repetitions of this process, often on a much larger scale than 

 in the blastoderm. The folds of the intestinal canal, the valvulae con- 

 niventes of its mucous membrane, the convolutions of the brain, and 

 the tubular windings of the perspiratory glands, with many other 

 analogous forms, are produced in a similar way. All these structures 

 are at first smooth or straight. They become thrown into folds or 

 convolutions during the development of the embryo, whenever they 

 grow more rapidly than the surrounding parts. 



Position of the Embryo in the Egg. Although the blastoderm is at 

 first apparently of uniform structure throughout, yet each particular 

 part has from the beginning a physiological individuality, which leads 

 to its subsequent development into a special organ or part of an organ. 



