No. 2.] DEVELOPMENT OF PETROMYZON". 27 1 



the stomodaeum be primitive, the connection between it and the 

 olfactory organ in the Cyclostomata is altogether unintelligible. 



With regard to the homologies of the pituitary body with 

 organs of invertebrates, the most probable suggestion seems to 

 be the one made by Balfour (4), Julin (20), and, latterly, by 

 Miss Sheldon (36) comparing it with the ciliated pit of the 

 Ascidians. Hubrecht's view (17) of homology with the pro- 

 boscis of Nemerteans may, perhaps, have some truth in it, but 

 the evidence is as yet very insufficient. At all events, I think 

 we may safely regard the hypophysis as an organ derived from 

 invertebrate ancestors, and as originally forming a cul-de-sac, 

 which opened upon the surface of the head and was in close 

 contact with the nervous axis. 



(/;) The Mid-Brain. — As Shipley has remarked, the line of 

 demarcation between the fore and mid-brains is for some time 

 obscure ; but it may, nevertheless, be unquestionably made out 

 even before the time of hatching. In horizontal sections (Fig. 

 23, PL X) there is a widening of the ventricle, produced by a 

 thinning of the walls at the plane where the fore-brain passes 

 into the mid-brain. At a later stage, when the bands of nerve 

 fibres make their appearance on the sides of the spinal cord, 

 hind and mid-brains, it is seen that they stop at the anterior 

 end of the latter, and do not form a definite envelope for the 

 fore-brain (Fig. 24). This condition persists for a considerable 

 period after hatching (Fig. ii, PI. IX); but now there is an 

 external constriction which shows the limits of the two regions 

 very clearly, and in larvae of 15 mm. length and upwards the 

 relations are perfectly plain (Fig. 15, PI. IX). The separation 

 between the mid and hind-brains is much more obviously indi- 

 cated from the first. Several observers, especially Shipley, have 

 noticed that in advanced embryos the roof of the entire brain 

 is in the median line only a single cell thick, and that at this 

 stage a dorsal fold of some depth occurs at the junction of the 

 mid and hind-brains. With the gradual correction of the cranial 

 flexure, already mentioned, this dorsal fold deepens and be- 

 comes more conspicuous. 



There has never been any difference of opinion as to the 

 dorsal limits of the mid-brain; but Ahlborn's account of the 

 structure in the adult calls for some examination. According 

 to this observer, the mid-brain cannot be regarded as having 



