ORGANIC CHEMISTRY AND CELL DIVISION 



107 



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 ; t., invagination furrow ; n., archen- 

 teron. (After Boas.) 



Fig. 38. Frog's Egg, 



Fig. 37. Diagrammatic Figures in Explanation of the Formation of the Showing Proportion- 



Third Germ Layer the Mesoderm. ate Increase of 



1, youngest, and 4, the oldest stage. Smaller Cells at 



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



References : 



E. W. MacBride, "Textbook of Embryology." 

 E. B. Wilson, "The Cell in Development and Inheritance/' 

 Kellicott, "Chordate Development." 

 Gurwitsch, "Morphologic and Biologic der Zelle." 

 Heidenhaim, "Plasma und Zelle." 

 Buchner, "Prakticum der Zellenleh're." 

 L. W. Sharp, "An Introduction to Cytology." 



W. E. Agar, "Cytology, with Special Reference to the Metazoan 

 Nucleus." 



L. Doncaster, "An Introduction to the Study of Cytology." 



