ORIGIN OF THE GERMINAL LAYERS. 



349 



blastica. In the case of other tissues there are a few instances 

 which might be regarded as examples of an organ primitively 

 developed in one of the two primary layers having become 

 secondarily carried into the mesoblast. The notochord has 

 sometimes been cited as such an organ, but, as indicated in a 

 previous chapter, it is probable that its hypoblastic origin can 

 always be demonstrated. 



FIG. 206. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.) 



A. Stage when the four hypoblast cells are nearly enclosed. 



B. Stage after the formation of the mesoblast has commenced by an infolding of 

 the lips of the blastopore. 



ep. epiblast; me. mesoblast; bl. blastopore. 



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 Cephalopoda, and some other 

 Mollusca, the evidence is in favour of its developing in the 

 mesoblast. Should future investigations confirm these conclu- 

 sions, a good example will be afforded of an organ changing the 

 layer from which it usually develops 1 . 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 



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 Mollusca. 

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

 me highly improbable. 



