GERMINAL LAYERS OF THE EMBRYO. 
263 
velopment of the generative organs in the Invertebrata is still 
very imperfectly known, but it is possible, that although 
usually developed in the mesoblast, they sometimes (e. g. 
Insecta) retain their primitive development from the epiblast. 
The nervous system, although imbedded in mesoblastic 
Fig. ^.—Epibolic Gastrula of Bonellia. (After Spengel.) A. Stage when 
the four Jiypoblast cells are nearly enclosed. B. Stage after the forma- 
tion of the mesoblast has commenced by an infolding' of the lips of the 
blastopore, ep. Epiblast ; me. mesoblast ; bl. blastopore. 
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 Cephalopoda, and some other 
Mollusca, the evidence is in favour of its developing in the 
mesoblast. Should future investigations confirm these con- 
clusions, a good example will be afforded of an organ chang- 
ing the layer from which it 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 constitute the 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 hypoblast.^ These facts are extremely in- 
‘ The reader is referred for this subject to the extremely valuable 
memoirs which have been recently published by the Hertwigs, especially 
(Ao. 13). He will find a general account of the subject written before 
the appearance of the Hertwigs’ memoir in p. 1 19 and 150 of volume I of 
my treatise on ‘Comparative Embryology.’ 
’ It would be interesting to know about the history of the various ner- 
vous structures found in the walls of the alimentary tract in the hio-lier 
forms. 1 have shown (‘ Development of Elasmobranch Fishes,’ p. 172) 
that the central part of the sympathetic system is derived from the epiblast. 
VOL. X.\. NEW SER. S 
