ORIGIN OF THE GERMINAL LAYERS. 



289 



The nervous system, although imbedded in mesoblastic derivates 

 in the adults of all the higher triploblastica, retains with marvellous 

 constancy its epiblastic origin (though it is usually separated from 

 the epiblast prior to its histogenic differentiation) ; yet in the Cepha- 

 lopoda, and some other MoUusca, the evidence is in favour of its 

 developing in the mesoblast. Should future investigations confirm 

 these conclusions, a good example will be afforded of an organ 

 changing the layer from which it usually develops \ The explanation 

 of such a change would be precisely the same as that already given 

 for the mesoblast as a whole. 



The actual mode of origin of various tissues, which in the 

 true triploblastic forms arise in mesoblast, can be traced in the 

 Coelenterata^ In this group the epiblast and hypoblast both give 

 rise to muscular and connective-tissue elements ; and 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 hypo- 

 blast ^ These facts are extremely interesting, but it is by no means 



Fig. 207. Two tkansvbrse sections through embryos of Hyurophilus 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. 



gr^f. germinal groove ; me. mesoblast; am. amnion; ylc. yolk. 



1 The Hertwigs hold that there is a distinct part of the nervous system which was 

 at first differentiated in the mesoblast in many types, amongst others the MoUusca. 

 The evidence in favour of this view is extremely scanty and the view itself appears to 

 me highly improbable. 



2 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. 149 and 150 of Volume i. of this treatise. 



■^ 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. 



B. E. II. 19 



