220 W. BALDWIN SPENCER. 



regarded as perfectly certain when its relationship to the 

 anterior end of the notochord is considered. 



In Petromyzon, according to Ahlborn/ the epiphysis 

 arises as a hollow outgrowth from the roof of the thalamen- 

 cephalon, which in subsequent development becomes divided 

 into three parts, (a) a proximal solid stalk, {b) two distal vesi- 

 cles of which the larger is the uppermost, whilst the smaller 

 acquires a secondary connection with the left ganglion habe- 

 nulse. The whole lies within the cartilages enclosing the brain, 

 and though a certain rod-like appearance is subsequently pro- 

 duced in the cells, particularly those of the upper vesicle, still 

 no pigment is developed and no differentiation into retina or 

 lens takes place. 



In Elasmobranchs^ the epiphysis stretches forward as a 

 hollow outgrowth with a dilated end, which may 

 remain within the skull cavity or be enclosed in the cartilage 

 of the roof. 



In Amphibia^ the same development takes place in early 

 stages, the organ remaining in Urodeles as a mushroom-shaped 

 structure, whilst in Anura it is differentiated into a vesicle 

 distally and a solid fibrous stalk proximally, the former being 

 afterwards cut off and occupying an extracranial position im- 

 mediately beneath the skin. 



In Reptilia it arises in all forms as a hollow, forwardly 

 directed outgrowth, which becomes most highly differentiated 

 in Lacertilia, where, in many forms, its distal vesicular por- 

 tion forms an optic organ. 



In Avesthe structure also stretches forward, originating as 

 a hollow outgrowth, and being subsequently divided into a 

 distal part which becomes vascular and a proximal solid stalk. 



In Mammalia, finally, the structure is much less developed, 

 the process being shorter than in the lower forms and directed 

 backwards. 



1 " Untersuchungen iiber das Gehirn der Petromyzon," ' Zeit. f. Wiss.,' 

 Bd. xxxix, p. 230, Tf. 13 and 16. 



2 Balfour, ' Elasra. Fishes,' p. 17. 



^ Henri de Graaf, op. cit., p. 23 (Urodeles) and p. 27 (Anura). 



