GENERAL FEATURES OF CHORDATE DEVELOPMENT 41 



Figure 8Z>. At the time of hatching, the yolk has been practically used up, and the 

 remnant is withdrawn into the embryo, the opening in the body wall where the yolk 

 stalk arises being then finally closed over. 



Obtain a slide bearing a cross-section through the trunk of a chick embryo of 

 two days' incubation. As explained above, the intestine of the chick embryo 

 is open below on the yolk. In making such sections the embryo is cut off from 

 the yolk, the lines of section being indicated in Figure 8C at x. After under- 

 standing the relation of embryo and yolk, examine the section with the low 

 power. The dorsal boundary of the section is a thin layer, the ectoderm, which 

 is slightly elevated in the median dorsal line; the ventral boundary is another 

 thin layer, the entoderm, which makes a slight upward bend in the median 

 ventral line, indicating the future intestine. In the median dorsal line just 

 beneath the ectoderm is the oval hollow section of the neural tube. Immediately 

 ventral to this is a small circular mass of cells, the notochord. On each side of 

 the neural tube is a squarish mass, its cells radiating from the center. This is 

 the epimere or mesoblastic somite. Lateral to the epimere and continuous with 

 it is a smaller mass, the mesomere or nephrotome, in which one or more tubules 

 with central holes are distinguishable. Beyond the mesomere the mesoderm is 

 observed to split into two layers. This region of the mesoderm is the hypomere 

 or lateral plate. The outer or dorsal layer of the hypomere is the somatic 

 mesoderm. It ascends and comes in contact with the ectoderm, the two together 

 constituting the somatopleure or body wall. The lower or ventral wall of the 

 hypomere is the splanchnic mesoderm; it descends and comes in contact with 

 the entoderm and the double layer thus formed is the splanchnopleure or intestinal 

 wall. The cavity between the somatic and splanchnic walls of the hypomere is 

 the coelom. As already explained, the hypomere in such embryos extends far 

 out over the yolk. Observe that the splanchnopleure contains many holes; 

 these are the cross-sections of blood vessels, which convey the food from the yolk 

 sac to the embryo. There is also a large artery in the embryo below each epimere. 

 Draw the section in diagram, coloring the three germ layers as before. 



F. THE FATE OF THE ECTODERM 



As we have seen, the ectoderm gives rise to the neural tube, from which 

 develop the brain, spinal cord, and nerves. The ectoderm also forms the external 

 layer of the skin and all of its derivatives, such as hair, nails, etc. It also gives 

 rise to the sensory part of all the sense organs, the lining membrane of the nasal 

 cavities, the mouth, and anus, the glands and other outgrowths of the nasal and 

 mouth cavities, the glands of the skin, the enamel of the teeth, and the lens of 

 the eye. 



G. THE FATE OF THE ENTODERM 



The entoderm is, as we have seen, the primitive intestine. This intestine 

 is the inner lining of the adult intestine. The entoderm thus forms the epithelial 



