Johnston, Morphology of the Head. 201 



I do not know of any evidence that this takes place in selach- 

 ians. The lower border of the neuropore lies a short distance 

 in front of the optic stalk, a vestige of which persists as the 

 praeoptic recess (recessus opticus) in the adult. The morpho- 

 logically anterior end of the brain in selachians is therefore 

 slightly in front of the praeoptic recess and probably behind the 

 anterior commissure. Ventral to this point an ectodermal pit 

 forms the hypophysis and lateral to the same point paired ecto- 

 dermal patches and corresponding expansions of the brain wall 

 adjacent to the neuropore form the nasal epithelia and olfactory 

 bulbs respectively. 



The developement in amphibia is exactly similar to that in 

 selachians. In Amblystoma piinctatuni the continuity of the 

 neural plate with slightly thickened ectoderm at the lower border 

 of the neuropore is present, the entoderm sends forward a 

 median solid prolongation in front of the mouth which meets 

 brain and ectoderm at their point of fusion and overlies the 

 slight depression from or in which the hypophysis is later formed. 

 The median mass of praeoral entoderm corresponds in all essen- 

 tials to the median, entoderm in selachians described above. In 

 Necturus, Miss Platt (108) describes a median mass of mesen- 

 toderm in the same position which she homologizes with the 

 median entoderm between the anterior head cavities in selach- 

 ians. His's figure of the frog embryo which shows the praeoral 

 entoderm in the same relations has already been referred to. 

 The thickened ectoderm forming the anterior limb of the hypo- 

 physial depression in Amblystoma is continuous laterally with 

 the nasal epithelium. When the hypophysial invagination 

 takes place it forms a slender pouch which presses upon the 

 ventral side of the praeoral entoderm slightly behind its tip. 

 All the relations are as in selachians except that the anterior 

 he;^d cavities are not formed and the praeoral entoderm is much 

 shorter. Since the relations just described are most clearly seen 

 at the time when the neuropore is closed and later, there can be 

 no question of the closing of the neuropore from below upwards. 

 Hence the point at which the lower border of the neuropore 

 meets the praeoral entoderm must be taken as the anterior end 



