366 THE NEW INDIVIDUAL Part IV 



and results in cells of unequal size. During these divisions the cells are gradu- 

 ally shifted outward and a temporary cavity is created in the center of the 

 cluster. Cells continue to divide until 200 or more are formed. The embryo is 

 then a hollow sphere called a blastula and the cavity within it is a blastocoel. 

 The cells of the embryo can easily interchange materials with the environment 

 of sea water. They have a relatively high income of oxygen and outgo of 

 excretory products resulting from the rapid metabolism of cell division espe- 

 cially at the animal pole. 



Gastrulation — Formation of Primitive Digestive Tract. Changes 

 now transform the hollow sphere of the blastula to the saclike form of the 

 gastrula (Gr., gaster, stomach) in which there is a new cavity, the archenteron, 

 or first digestive tract (Fig. 19.5). 



In the early part of gastrulation, the embryo is a double layered cup such 

 as a soft rubber ball would be if you pressed your thumb into its side. The side 

 forced in would be comparable to endoderm and chordamesoderm, and the 

 dent to the cavity of the archenteron; the other side of the ball would be ecto- 

 derm and the cavity inside the ball being pushed out of existence, the blasto- 

 coel. The archenteron appears gradually foreshadowed by the differentiation 

 of cytoplasm in the fertilized egg and the shape of the blastula (Fig. 19.5). 

 Toward the end of the blastula stage the vegetal region begins to flatten ever 

 so slightly like one side of a waning moon. Hindered by their content of yolk, 

 the cells on the flattened side divide fewer times and thus are larger than the 

 others. Presently the flat region is turned inward or invaginated more and 

 more sharply. 



The embryo is now shaped like a raspberry. The opening into the archen- 

 teron gradually becomes smaller due to the multiplication and inturning of 

 cells about its rim and finally becomes the minute blastopore. Its rim is the 

 transition zone between endodermal cells and chordamesoderm, and the ecto- 

 derm. The endoderm will line the digestive canal. The chordamesoderm will 

 make the notochord and the mesoderm that forms the bulk of the body, of 

 organs such as the liver, the lining of the body cavity, and all muscles and 

 bone. The ectodermal cells are the ancestors of the cells of the nervous system 

 and outer layers of skin. The rim of the blastopore is the germ ring, a growth 

 zone in which cells form rapidly especially in the important side, called the 

 dorsal lip of the blastopore. This is the starting place of the notochord, present 

 throughout life in the ancestors of vertebrates and the forerunner of the back- 

 bone in the early embryo and the mesoderm of every vertebrate from fish to 

 man. The development thus far occurs within about seven hours after the fer- 

 tilization of the egg. As in all eggs, it varies with the temperature and other 

 conditions. 



Nervous System — Notochord and Mesoderm. As the archenteron con- 

 tinues to enlarge, the dorsal surface of the embryo flattens and a broad band 



