ONTOGENESIS 207 



The primitive embryo thus comes to consist of two 

 germinal layers, ectoderm and entoderm, forming a 

 concavo-convex plate, the convex surface of which con- 

 sisting of ectoderm is destined to form the dorsum and 

 external coverings of the embryo, the concave surface 

 the internal organs about which the embryo is to grow. 

 As the growth proceeds the circumference of the germinal 

 disc increases and the subjacent yolk is absorbed as it 

 furnishes the growing cells with nourishment. The 

 increasing disc does not grow uniformly and hence be- 

 comes thrown into folds which, when viewed from the 

 dorsal surface, indicate the position of future structures. 

 Thus an anterior transverse fold indicates where the am- 

 niotic membrane is to form; a second transverse fold, 



FIQ. 89. Cross-section through the middle of the primitive streak of a chick's 

 germ disc. At some distance from the primitive groove is to be seen upon the 

 left side of the figure in cross-section the marginal groove of His; upon the right 

 side it is as yet little developed, ak, Outer, ik, inner, mk, middle germ layers; 

 pr, primitive groove; ps, primitive streak; gr, marginal groove. (After Roller.) 



where the head of the embryo is to develop. The longi- 

 tudinal groove in the anterior part of the disc indicates 

 where the spinal cord will form, and the folds on each 

 side of the groove two arches of the ectoderm by con- 

 crescence or fusion, of which the spinal canal will event- 

 ually be formed. 



While these wrinkles or folds are preparing the way 

 for future development, the mesoderm, from which the 

 skeletal, motor, circulatory, and connective tissues are to 

 form, is being prepared by the penetration into the space 

 between the ectoderm and entoderm of a mass of small 

 cells in many superimposed layers, arising from both 

 ectoderm and entoderm, and as these cells become in- 

 closed between the two chief blastodermic layers they 



