188 ¥. M. BALl'OUR AND F. DEIGHTON. 



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 an- 

 ticipated 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 regarding 

 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 Amphioxus 

 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 the fusion of 

 the layers at the front end of the primitive streak is the 

 universal fusion of the layers at the dorsal lip of the blastopore, 

 which is so well known in the lower vertebrate types. 



