THE DEVELOPMENT OP THE MOLE. 151 



The Medullary Canal. — The closure of the medullary groove 

 commences in the region of the first protovertebra during 

 Stage G and proceeds anteriorly and posteriorly, and at the 

 close of Stage j a complete canal is formed as far back as the 

 last (fourteenth) protovertebra. The lateral walls of the canal 

 thicken, and are converted into an hour-glass form in places. 

 The migration of mesoblast (nutritive) cells into the walls of 

 the canal is noted in Stages h and j. 



The Brain. — The three divisions of the brain are indicated in 

 Stage J, and a well-marked cranial flexure is then present. 

 The infundibulum is just apparent at this stage in close con- 

 nection with the front end of the alimentary canal and noto- 

 chord (fig. 49). 



The Optic Vesicles are formed from the optic grooves by the 

 closure of the medullary canal. These organs first appear 

 extremely early, but their development is soon checked, doubt- 

 less in consequence of the habits of the adult animal. 



The Ear in Stage j is merely indicated as a deep groove in 

 a thickened mass of mesoblast on either side of the hind-brain. 



The Cranial and Spinal Nerves are not described. 



The Hypoblast may be divided into axial and peripheral 

 portions. The peripheral hypoblast, a single layer of flattened 

 cells, extends on all sides over the embryonic area during 

 Stage E. The deepening of the medullary groove stretches 

 these cells and flattens them still more, but the thickening of 

 the lateral mesoblast forces the lateral hypoblast down, removes 

 the strain, and its cells become rounded. 



The Notochord is formed of axial hypoblast cells. In Stage 

 c a mass of axial hypoblast cells are continuous with two 

 lateral masses of mesoblast — derived from lateral hypoblast — 

 and with the lateral hypoblast layer also. In Stages d and e 

 the axial mass becomes isolated from the lateral mesoblast 

 plates, and gradually decreases in size below the deepening 

 medullary groove until in that portion where the groove is 

 deepest, i. e. near the centre, a single layer of flattened cells is 

 all that exist. 



It does not, however, become reduced to this extent through- 



