238 



THE CONTINUITY OF THE RACE 



Fig. 15.16. Embryonic development of the chick. A, diagrammatic section through the 

 lien's egg, showing relation of "yolk" (the true egg), white, and the shell and its membranes. 

 Many details of structure are omitted. Note particularly the blastoderm (blastodisk), a 

 small polar disk of cytoplasm enclosing the nucleus. The blastoderm alone undergoes cleav- 

 age and forms the embryo and its membranes. The huge mass of yolk is inert food material, 

 which with the added white is used as a source of energy and raw materials for development. 



B, the blastoderm. This is the blastodisk, now seen from above, after some hours of 

 development while the egg is still in the hen's body. Cleavage has taken place, and first a 

 blastula and then a gastrula have been formed. The blastula had the shape of a flat cap or 

 plate of cells, and the gastrula was produced by the cell plate turning under at one point (the 

 posterior end) and growing forward into the shallow space between the yolk and the upper 

 layer. This made the blastoderm two-layered, with an upper ectoderm and a lower ento- 

 derm. Note the clearer inner space (area pellucida) in which the embryo proper will develop, 

 and the outer region (area opaca) which will form part of the extraembryonic membranes. 



C, the embryo after about 24 hours of incubation, viewed from above. Except for the 

 clear space in front of the head there are now three layers of tissue — ectoderm on top, ento- 

 derm next to the yolk, and between them (except along the midline) a mesoderm which is in 

 part split into two layers. The notochord lies between the right and left plates of mesoderm. 

 Note the four pairs of somites formed from the denser part of the mesoderm, and the thick- 

 ening neural ridges of ectoderm. 



D, the embryo after 33 hours of incubation, viewed from above. Because the tissues are 

 transparent, one sees both outer and inner structures. A neural tube has now formed by 



