868 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 



that it unites posteriorly with a forward growth of the axial 

 tissue of the primitive streak, while it is laterally continuous, at 

 first, both with the mesoblast of the lateral plates and with the 

 hypoblast. At a later period its connection with the mesoblast 

 is severed, while the hypoblast becomes differentiated as a con- 

 tinuous layer below it. 



As to the hypoblastic origin of the notochord, we are again 

 in complete accord with Gerlach ; but we differ from him in 

 admitting that the notochord is continuous posteriorly with the 

 axial tissue of the primitive streak, and also at first continuous 

 with the lateral plates of mesoblast. 



The account we have given of the formation of the mesoblast 

 may appear to the reader somewhat fantastic, and on that ac- 

 count not very credible. We believe, however, that if the view 

 which has been elsewhere urged by one of us, that the primitive 

 streak is the homologue of the blastopore of the lower ver- 

 tebrates is accepted, the features we have described receive an 

 adequate explanation. 



The growth outwards of part of the mesoblast from the axial 

 line of the primitive streak is a repetition of the well-known 

 growth from the lips of the blastopore. It might have been 

 anticipated that all the layers would fuse along the line of the 

 primitive streak, and that the hypoblast as well as part of the 

 mesoblast would grow out from it. There is, however, clearly a 

 precocious formation of the hypoblast ; but the formation of the 

 mesoblast of the primitive streak, partly from the epiblast and 

 partly from the hypoblast, is satisfactorily explained by re- 

 garding the whole structure as the blastopore. The two parts 

 of the mesoblast subsequently become indistinguishable, and 

 their difference in origin is, on the above view, to be regarded as 

 simply due to a difference of position, and not as having a deeper 

 significance. 



The differentiation 'of the lateral plates of mesoblast of the 

 trunk directly from the hypoblast is again a fundamental feature 

 of vertebrate embryology, occurring in all types from Am- 

 phioxus upwards, the meaning of which has been fully dealt 

 with in the Treatise on Comparative Embryology by one of us. 

 Lastly, the formation of the notochord from the hypoblast is 

 the typical vertebrate mode of formation of this organ, while 



