286 



THE PROTOCHORDATA. 



(where there is no internal opening of the hypophysis 

 into the pharynx), the external opening of the hypophysis 

 does not close up, as in the higher forms, but persists 

 throughout life, becoming carried round to the top of 

 the head during the embryonic development by differ- 

 ential growth of neighbouring parts, as has been actually 

 observed in Petromyzon. 



et 



irn. 



Fig. 135. Median sagittal section through the head of young Ammocostes. 

 (After KUPFFER.) 



The arrow indicates the extent to which the hypophysis has been (hypothetically) 

 removed from the neighbourhood of the neuropore (lobus olfactorius impar). 



ch. Notochord. ec. Ectoderm, en. Endoderm. ep. Epiphysis. hy. Hypo- 

 physial involution, l.o. Lobus olfactorius impar. n. Nasal involution, pm. Me- 

 dian portion of praemandibular cavity, st. Stomodoeum. F.M.H. Primary fore-, 

 mid-, and hind-brain. 



In other cases, as, for example, in the embryo of the 

 rabbit, it has been observed that the hypophysis actually 

 undergoes a temporary fusion with the front end of the 

 notochord ; and in all cases the distal end of the hypophysis 

 grows inwards as much towards the notochord as towards 

 the infundibulum, so that for the embryonic stages of the 

 craniate Vertebrates it might be said that the relations of 



