Introduction to the Coelomata 261 



Mesenchyme arises from different germ-layers in different phyla of 

 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 chiefly 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 

 vertebrata are such organs as the lungs, liver, pancreas, and urinary blad- 

 der. 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 

 " ^wausitsr layers are alike in that they have both con- 



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

 h y Sa)° £ id^rrSdSSS."^ * will be remembered, is but a single layer 

 latter forms a tube within a tube. of cells forming a more or less spherical 



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

 stomodeurn ( ), 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 comes 

 another infolding of ectoderm in the gastrula at the opposite end from 

 the stomodeurn. This forms an anal opening which is called a procto- 

 deum ( ). 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 

 mechanical injury, such as pressure or laceration, cannot affect the body 

 unless it strikes the outer portions first. Therefore, the sensory nerve 

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

 they can receive the message of threatened danger first. These danger 

 messages are then carried to the central nervous system where a coordi- 

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



