Chemistry of Living Matter and Cell Division 



107 



( ), but if it becomes a single layer of cells and is 



hollow, it is known as a blastula ( ). In the latter 



case the hollow portion in the center is rilled with a fluid. The hollow 

 space itself is called the segmentation cavity. 



If this blastula indents (just as though one were to take a hollow 

 rubber ball and push in one side with a ringer), there are two layers 

 in the indented region. The outer layer is called the ectoderm or epiblast, 

 and the inner the entoderm, endoderm, or hypoblast, while the entire two 

 layered mass is known as a gastrula ( ). The 



indentation is also called invagination and gastrulation (Fig. 36). 



Having indented, the indented portion draws together to form a 



single mouth-like opening. This open- 

 ing is the blastopore ( ), 

 and the newly made cavity surrounded 

 by entoderm is the primitive intestinal 

 tract or archenteron ( ). 

 In our study of the hydra it will be 

 found that that animal grows thus far 

 and then remains throughout its entire 

 career in the gastrula stage. 



In higher forms a third layer is 

 formed between the ectoderm and en- 

 toderm known as the mesoderm. Ani- 

 mals having these three germ layers 

 (Fig. 37) are called triploblastic 

 ( ). All tissues 



and organs are derived from some one 

 or more of these germ layers. To 

 study this development is the special 

 province of Embryology. 



Often certain blastomeres grow 

 more rapidly than others in the same 

 embryo. Such is the case with frog's eggs (Fig. 38). This results in 

 the more rapidly growing cells surrounding those which divide more 

 slowly. A growing of one set of cells over another is called epibole 

 ( ). The separation of the germ layers or mem- 



branes by splitting apart is known as delamination. 



Fig. 36. Formation of the Gastrula in 

 Amphibia. Diagrammatic Longi- 

 tudinal Section. 

 1, Blastula; 2, the invagination has be- 

 gun at i (the corresponding place in 1 is 

 indicated by an arrow) ; the invagination 

 is in the form of a furrow, but does not 

 yet surround the egg; 3, the invagination 

 is proceeding; 4, perfect gastrula; the 

 archenteron is almost filled with a project- 

 ing part of the hypoblast, which is later 

 dissolved and absorbed by the embryo, ek., 

 ectoderm (light) ; en., entoderm (shaded) ; 

 g., mouth of gastrula; h., segmentation 

 cavity; %., invagination furrow; »., archen- 

 teron. (After Boas.) 



Fig. 37. Diagrammatic Figures in Explanation of the Formation of the 



Third Germ Layer — the Mesoderm. 

 1, youngest, and 4, the oldest stage. 



ek., ectoderm; en,, endoderm; m,, mesoderm, (After Boas). 



Fig. 38. Frog's Egg, 

 Showing Proportion- 

 ate Increase of 

 Smaller Cells at 

 Top of Egg. 



