350 



ORIGIN OF THE MESOBLAST. 



Ccelenterata 1 . In this group the epiblast and hypoblast both 

 give rise to muscular and connective-tissue elements ; and 



o * 



although the main part of the nervous system is formed in the 

 epiblast, it seems certain that in some types nerves may be 

 derived from the hypoblast' 2 . These facts are extremely interest- 



FlG. 207. TWO TRANSVERSE SECTIONS THROUGH EMBRYOS OF HYDROPHILUS 



PICEUS. (After Kowalevsky.) 



A. Section through an embryo at the point where the two germinal folds most 

 approximate. 



B. Section through an embryo, in the anterior region where the folds of the 

 amnion have not united. 



gg. germinal groove; me. mesoblast; am. amnion; yk. yolk. 



ing, but it is by no means certain that any conclusions can 

 be directly drawn from them as to the actual origin of the 

 mesoblast in the triploblastic forms, till we know from what 

 diploblastic forms the triploblastica originated. All that they 

 shew is that any of the constituents of the mesoblast may have 

 originated from either of the primitive layers. 



1 The reader is referred for this subject to the valuable memoirs which have been 

 recently published by the Hertwigs, especially to No. 270. He will find a general 

 account of the subject written before the appearance of the Hertwigs' memoir in 

 pp. 180-182 of Volume II. of this treatise. 



2 It would be interesting to know the history of the various nervous structures 

 found in the walls of the alimentary tract in the higher forms. I have shewn 

 (Development of Elasmobranch Fishes, p. 172) that the central part of the sympa- 

 thetic system is derived from the epiblast. It would however be well to work over 

 the development of Auerbach's plexus. 



