260 GENERAL BIOLOGY 



animals. It may arise from the entoderm or ectoderm or both, or even 

 from the walls of the coelom. In this latter case it may spring from 

 ectoderm, entoderm, and mesoderm. In the higher coelomata it arises, 

 however, "partly from the ectoderm but principally from the outer wall 

 of the coelom. Everywhere it gives rise to connective tissue and to the 

 tissues developed from this (tendon, cartilage; bone, etc.), whereas the 

 coelomic wall or true mesoderm gives rise to the generative cells and 

 their ducts, and the main parts of the muscular system, including the 

 muscular coats of the principal blood-vessels. 



The entoderm, after the mesoderm has separated from it, forms the 

 lining of the digestive tube and of its appendages, which in the higher 

 Vertebra.ta are the organs known as lungs, liver, pancreas, and urinary 

 bladder. The basis of the skeleton of Vertebrata, the gelatinous rod 

 called the notochord, also arises from the entoderm. 



After gastrulation has taken place in the growing embryo, there are 

 only two germ layers, ectoderm and entoderm. The inner layer under- 

 goes various changes, as it is to be used for 

 a totally different purpose from its outer 

 protective layer. It must be remembered, 

 however, that just after indentation, both 

 Fig 164 layers are alike in that they have both con- 



Diagrammatic cross section of the stituted the simple blastula. The blastula, 



body of a coelenterate (such as the . 



hydra) and of a coeiomate. The it will be remembered, is but a single layer 



latter forms a tube within a tube. r ,, .. , 1-1 



of cells forming a more or less spherical 



body. The opening formed by gastrulation and known as the mouth or 

 stomodeum ( ) does not undergo the same change that does the 



part on the more interior portion of what is now called the entoderm. 

 In fact, the mouth region remains ectodermal. As soon as an organism 

 has formed three germ-layers and has both an opening in its body for 

 ingestion as well as egestion of food, there com.es another infolding of 

 ectoderm in the gastrula at the opposite end from the stomodeum. This 

 forms an anal opening which is called a proctodeum ( ) 



This infolding, just as the stomodeal infolding, is also ectoderm. 



It is of interest and value here to know that the entire brain and 

 nervous system arise from ectoderm. It will be readily understood why 

 this is so, when it is realized that no organism from the simplest flower 

 up to man, could possibly live unless there were some method by which 

 such organism could protect itself when danger threatened. Any me- 

 chanical injury, such as pressure or laceration, cannot affect the body un- 

 less it strikes the outer portions first. Therefore the sensory nerve end- 

 ings must be placed close to the outer portion of the body so that they 

 will receive the message of threatened danger first. These danger mes- 

 sages are then carried to the central nervous system where a co-ordina- 

 tion must be brought about between the sensory fibers and the motor 



