Reproduction 



251 



importance is that it gives the embryonic material increased superficial 

 contact with the environment, thus favoring metabolism. Its prospec- 

 tive significance lies in the fact that further development is to consist, 



a' 



Fig. 208. Cleavage of the germ-disk 

 of the egg of a turtle (Glyptemys in- 

 sculpta); eight-cell stage. The eggshell 

 is not shown. About twice natural size. 

 (A) Albumen; (C) the eight-cell blasto- 

 derm; (Y) yolk. (After Louis Agassiz. 

 Courtesy, Neal and Rand: "Chordate 

 Anatomy." Philadelphia, The Rlak- 

 iston Company.) 



to a large extent, in the manipulation of layers of embryonic material. 

 The adult is hollow. It has a body-cavity and other cavities. Most of its 

 organs are hollow. The walls of the hollow structures are constituted 



Fig. 209. Early blastoderm of chick; plane of section 

 passes through center of egg. (R) Rlastocoele (subgermi- 

 nal or cleavage cavity); (C) cells of blastoderm; (V) 

 fluid-filled vesicles; (Y 1 ) yellow yolk; (Y 2 ) white yolk. 

 (Magnified nearly 20 diameters.) (After Duval. Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, 

 The Rlakiston Company.) 



of layers — skin, epithelium, endothelium, peritoneum, muscle layers, 

 connective-tissue layers. For the construction of such a many-layered 

 thing, the embryo naturally proceeds as early as possible to dispose its 

 building material in the form of layers. 



