THE DEVELOPMENT OF THE MOLE. 135 



The hooked anterior end of the notochord is due to its origin 

 from the front wall of the fore-gut. Its close approximation 

 to the fore-brain is noted. 



The relatively small size of the notochord to the nervous 

 system in the Mole is pointed out, and it is suggested the early 

 development of the latter is the cause of the check adminis- 

 tered to the growth of the former, a check from which it appears 

 never entirely to recover. 



The Alimentary Canal first appears in Stage d as a short 

 tubular diverticulum, projecting below the cephalic plate nearly 

 to the anterior end of the embryo. 



The tube enlarges and extends backwards during the pro- 

 gress of the folding off of the embryo during Stages e to j, 

 and the cranial flexure causes a ventral enlargement, which 

 is somewhat posterior to the original anterior diverti- 

 culum. 



The mouth and the visceral clefts are not formed at the close 

 of Stage J, but the epiblast and hypoblast have fused at the 

 point where the mouth will eventually be formed, and several 

 lateral outgrowths from the now widened fore-gut exist ; in the 

 case of one of these, the anterior one, the hypoblast has reached 

 the epiblast, and the two layers are partially fused at that 

 point. 



The mouth is formed at the apex of a Y-shaped groove, the 

 diverging limbs of which are directed forwards ; these grooves 

 are the anterior border of the first visceral arch. 



The primary anterior diverticulum would indicate the 

 existence primitively of a terminal mouth, while the two 

 grooves, at the junction of which the mouth is formed, 

 would suggest a paired origin for the existing mouth of the 

 animal. 



The Mesoblastic Somites and Body Cavity. — The lateral plates 

 of mesoblast are split horizontally into somatic and splanchnic 

 layers, but the split is not actually carried through both peri- 

 pheral and axial portions of the plates, being merely indicated 

 in Stage e in the axial portion. The mesoblast of the head 

 also is not split, and no cavity is formed there. 



