THE DEVELOPMENT OF THE MOLE. 139 



greater or less substantiality. It will be noticed the knobs 

 are more or less free from the underlying flattened hypoblast 

 cells, while in the median line there are no flattened cells, thus 

 showing the process of the growth of the lateral hypoblast 

 below the axial primitive hypoblast. 



The relation of the notochord at the front end of the embryo 

 requires special notice ; it will be best understood by a reference 

 to figures of longitudinal sections through embryos of Stage 

 E (fig. 11), Stage H (fig. 34), and Stage j (fig. 49). In 

 fig. 11 the notochord is not separated from the roof of the 

 fore-gut; in fig. 34 it remains attached to the anterior wall 

 of the fore-gut, although isolated posteriorly ; but in fig. 49 the 

 notochord, although joined at its anterior extremity to the 

 hypoblast, is separated from it throughout its extent posteriorly. 



The hooked anterior end of the notochord, so characteristic 

 of this organ, is seen to be due, in the Mole, to the fact that it 

 is derived from the anterior wall of the alimentary tract after 

 the cranial flexure has commenced. 



At the close of Stage j, therefore, the notochord is continu- 

 ous with the epiblast at the front end of the embryo, by means 

 of the front wall of the fore-gut, which is fused with the 

 epiblast at the point where the mouth will eventually be 

 formed ; and posteriorly, at the anterior end of the primitive 

 streak, where epiblast, hypoblast, and mesoblast are all joined 

 together (compare figs. 49 and 50). 



The close relation of the fore-brain to the notochord, a rela- 

 tion brought about not so much by the cranial flexure as by 

 the ventral enlargement of the brain at this point, will be 

 referred to in another communication, which I hope shortly to 

 make, upon the pituitary body of the Mole. 



There is one other point of interest in the growth of the 

 notochord in the Mole, and that is its size compared with the 

 nervous system. The relative size of the notochord compared 

 with the nervous system is less in the higher than it is in the 

 lower Vertebrate embryos. In the Mole the notochord is rela- 

 tively smaller than it is in any other Vertebrate embryo I am 

 acquainted with, and it appears to me the reduction in size is 



VOL. XXVII, PART 2. NEW SER. L 



