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THE AMERICAN NATURALIST [Vol. XLII 



sponge represents the first step in the differentiation of 

 the neuro-muscular mechanism, a step in which the pri- 

 mary and fundamental character of muscle is disclosed in 

 that this is the only constituent present. This self- 

 sufficiency of muscle is also made evident in the rhythmic 

 beat of the embryonic chick heart before nervous differ- 

 entiation has taken place. 



In the sponges the primitive muscle cells lie either in 

 the ectoderm or the subjacent mesoderm. In the coral 

 animals, the jellyfishes, and their allies, the muscle cells 

 occur in the deeper parts of the ectoderm and entoderm, 

 a position where a forming mesoderm might be expected. 

 It is usually stated that in the higher animals the muscles 

 are derived from the mesoderm, and in individual devel- 

 opment this is true, but from a phylogenetic standpoint 

 I believe the reverse to be the case, namely, that the meso- 

 derm has come from muscle and that the first step in the 

 real origin of this layer is indicated in the migration of 

 the muscular tissue of the ccelenterates or other like forms 

 from the ectoderm or entoderm into the region between 

 these two layers. This operation, which involves both 

 layers in many of the lower animals, is usually limited in 

 the higher forms to the entoderm, probably because this 

 layer is the one which by reason of its proximity to the 

 digestive cavity can best supply material for future 

 growth. But, however this may be, it seems to me prob- 

 able that the mesoderm has had its origin in the process 

 of muscle formation, a process that is seen in its incip- 

 iency in the ccelenterates. That the mesoderm is also 

 concerned with providing mechanical support for the ani- 

 mal is obvious, but in my opinion its contractile function 

 is the primary one. 



It cannot be stated with certainty at present that in 

 the normal action of ccelenterate muscle this tissue is 

 stimulated directly, though the investigations of Loeb 

 on the jellyfish, Gonionemus, show beyond a doubt that 

 such a method of stimulation is possible. It is, however, 

 well established that in many jellyfishes muscle action is 



