THE PITUITARY BODY. 413 



The uppermost end of this does not till near the close of 

 stage K become markedly constricted off from the remainder, 

 but is nevertheless the rudiment of the pituitary body. PL 15, 

 figs. 9 a and 12 m shew in a most conclusive manner the cor- 

 rectness of the above account, and demonstrate that it is^from 

 the mouth involution, and not, as has usually been stated, from 

 the alimentary canal, that the pituitary body is derived. 



This fact was mentioned in my preliminary account of Elas- 

 mobranch development 1 ; and has also been shewn to be the 

 case in Amphibians by Gotte 2 ; and in Birds by Mihalkowics 3 . 

 The fact is of considerable importance with reference to specula- 

 tions as to the meaning of this body. 



Plate 15, fig. 7 represents a transverse section through the 

 head during a stage between I and K ; but, owing to the cranial 

 flexure, it cuts the fore part of the head longitudinally and hori- 

 zontally, and passes through both the fore-brain (fb) and the 

 hind-brain (iv. v.}. Close to the base of the fore-brain are seen 

 the mouth (m), and the pituitary involution from this (pt). In 

 contact with the pituitary involution is the blind anterior ter- 

 mination of the throat, which a little way back opens to the 

 exterior by the first visceral cleft (l. v.c.}. This figure alone 

 suffices to demonstrate the correctness of the above account of 

 the pituitary body ; but the truth of this is still further con- 

 firmed by other figures on the same plate (figs. 9 a and 12 m] ; 

 in which the mouth involution is in contact with, but still 

 separated from, the front end of the alimentary tract. By the 

 close of stage K, the septum between the mouth and throat 

 becomes pierced, and the two are placed in communication. 

 This condition is shewn in PI. 15, fig. i6a, and PI. 16, figs, i a, 

 i c, pt. In these figures the pituitary involution has become 

 very partially constricted off from the mouth involution, though 

 still in direct communication with it. In later stages the 

 pituitary involution becomes longer and dilated terminally, 

 while the passage connecting it with the mouth becomes nar- 



1 Quarterly Journal of Microscopic Science, Oct. 1874. 



' 2 Ent-wicklungsgeschichte der Unke. Gotte was the first to draw attention to this 

 fact. His observations were then shewn to hold true for Elasmobranchs by myself, 

 and subsequently for Birds by Mihalkowics. 



:! Arch. f. mift: Anat. Vol. XI. 



