HOMOLOGY. 



3 



Beneden*, referring to the body in Tunicaries (Savigny's 

 tubercle," Hancock's and Ussow's " olfactory organ "), 

 which is homologized by M. Julinf with the " pituitary 

 gland " of Vertebrates, compares it to the kidney, and holds 

 that by a communication with the " peribranchial cavity" of 

 the Ascidian it discharges its urinary excretion therein. 

 Lacaze-Duthiers describes a similar, or the same, " glandular 

 organ " in juxtaposition with the ciliated sac in the Simple 

 Ascidians"J. Balfour deems it ''possible that this organ, 

 as well as the ciliated sack, may be related to the pituitary 

 body"§. Of the function of that body in the Vertebrate 

 series the latter embryologist entertains a view "as possible," 

 viz., that " the pituitary body is a glandular structure, which 

 originally opened into the mouth in the lower Chordata " 

 (syn. Vertehrata), "but which has, in all existing forms, 

 ceased to be functional " 1| . The alternative view is as 

 follows : — " The true nature of the pituitary body has not 

 yet been made out. It is clearly a rudimentary organ in 

 existing craniate Vertebrates, and its development indicates 

 that, when functional, it was probably a sense-organ opening 

 into the mouth 



The impression left by results of observations, presently to 

 be submitted, leads me to define the pituitary body rather as 

 a " residuary " than a " rudimentary " organ. At least I 



* Archives de Biologie, 8vo, 1881, torn. ii. fascicule ii. p. 230. 



t lUd. fascicule i. p. 59 tt seq^. (I may remark that, regarding the 

 eylindroid shape as well as position of the neural centre in some Ascidians, 

 I have viewed it as the homologue of the same part in Amphioxus, and 

 the co-extensive body beneath as indicative of the notochord.) 



+ " Les Ascidies simples des Cotes de France," Archives de Biologie 

 experimentale et generale, vol. iii. 1874, p. 329. 



§ Treatise on Comparative Embryology, 8vo, 1881, vol. ii. p. 360. 



II Ih. ih. p. 360. % 2b. ib. p. 359. 



b2 



