AunusT 17, 1SS3.] 



SCIENCE. 



185 



tiikcn from mv article in tlio Morphol. jnJirb., 

 vii). The upper lip is soinowhat roimdod in 

 longitiuliiial section, and hoinuled anteiioilv by 

 a very slight depression, whicii is the hcjjinning 

 of tlie pituitary Ijody ; but, as tliis is also the 

 beginning of the invagination to form the nasal 

 sac' I have preferred not to separate them, as 

 Dohrn has done. In the next stage (fig. 2) 



Fio. 2. — Sec-lion through head of an older embryo just before 

 hatcliing. U(/', olfuclury epithelium. Oilier lellvrs aa in lig. 1. 



the nasal epithelium has become much thick- 

 ened, the pituitary involution deeper, and the 

 upper lip elongated so as to become triangular 

 in section. At tliis time the cranial flexure 

 has reached its maximum ; though it is ftir less 

 than in most other groiijjs, owing to the rela- 

 tively small size of the fore and mid brains. 

 The moutli is ventral in jiosition, corresponding 

 very closelv to the selachian mouth in position 

 and shape. 



Shortly after this, the upper lii) begins that 

 remarkable series of transformations to wliich, 

 as I long ago pointed out, man}- of tlie most 

 striking peculiarities of the cyclostome organi- 

 zation are due. The posterior edge of tiie lip 

 elongates rapidly, becoming triangular in sec- 

 tion ; while the whole anterior part of the head 

 rotates forwards, tiius tending to correct the 

 cranial flexure, and bringing the mouth to point 

 somewhat forward as well 

 as downward. By this 

 process the edge of the 

 lip, which in fig. 2 is 

 directed backwards, now 

 comes to point down- 

 wards (fig. 3) ; at the 

 same time, the opening 

 of the nasal pit points 

 forwards instead of down- 

 wards. The involution 

 for the nasal passage and 

 pituitary body has now become a long tube of 

 cells, which transverse sections show us to be 



[o. 3. — Section Ihroiitfh 

 head of a very ymni^ 

 larva of the lamprey. 

 Ijutiern aa before. 



1 Ry natial sac. I mean the blin 

 (Vo.m (be olfactory epithelium. 



pneenKC, as dintinguished 



perforated by a small lumen. The end of this 

 cellular tube reaches to the infnndibulum, with 

 which it lies in close contact. This portion 

 will give rise to the pituitary body. Up to this 

 time there has Ijoen no line of separation be- 

 tween the pituitary involution and the nasal 

 e|)ithelium ; but when the (jrocess of rotation 

 of the ui)per lip, and corroi-tion of the cranial 

 tlixnre, is coni|ileted, the edge of the lip 

 points directly forward, having i)assed through 

 an angle of ISO", and the opening of the nasal 

 sac is on the doi'sal instead of the ventral sur- 

 face of the head. At this time a fold appears 

 below the olfactoi-y epithelium, separating it 

 distinctly from the pituitary passage. 



The pituitary body is formed from part of 

 the epithelium of this passage, and consists 

 ol' solid follicles, separated by connective tissue. 

 According to Dohrn (loc. cit. , p. 1 TS) , this body 

 is not constricted off from the passage or nasal 

 sac at any time during larval life. I have 

 not been able to satisfy myself, as yet, upon 

 this point; but I am not inclined to agree 

 with this view. 



As to the morphological significance of the 

 pituitai-y body, many views have been pro- 

 pounded, some of them Ijearing upon the ques- 

 tion of the origin of the vertebrates. Some 

 writers have contended that the conario-hypo- 

 physial tract through the brain is the remnant 

 of the old mouth and gullet, which, in the ances- 

 tors of the vertebi-ates, passed through a ring of 

 nervous tissue, as in the annelids. Space will 

 not permit a discussion of this hypothesis : nor 

 is such discussion necessary, as Balfour (Elas- 

 mobranch fishes, p. 170) has stated the insu- 

 perable objections to the view. Dohrn, in the 

 pamphlet already quoted, adopts a view some- 

 what like one originally proi)ounded by GiJtte, 

 and adds a suggestion of his own. lie consid- 

 ers the entire blind nasal sac of the lampi-ey 

 to belong to the pituitary body, and that this 

 sac has arisen fi'om the coalescence of a pair 

 of gill-slits. This hypothesis is but the cairy- 

 ing-out of the theory- so ably advocated in the 

 very suggestive pamphlet ' Ueber den ursprung 

 (Icr wirbcllhiere.' But. until it can be shown 

 that the vertebrate mouth is a new formation, 

 the existence of pi-e-oral gill-clefts hardly mer- 

 its discussion. I reserve for a later paper the 

 ccnisideration of the origin of the vertebrate 

 mouth, — a question which is the lur4iing-point 

 of the solution of all these problems. 



Balfom- has suggested an explanation of the 

 pituitary body. "It is," he says (p. . "Jo!*), 

 " dearly a rudiraentai-y organ in existing crani- 

 ate vertebrates ; and its development inilicates, 

 that when functional it was probably a .sense- 



