No. 2.] DEVELOPMENT OF PETROMYZON: 289 



enlarged, both in length and in diameter. It apparently 

 functions as an apparatus for drawing water over the sensory 

 epithelium, and so compensates for the removal of the olfactory 

 organ from the region of the mouth. 



An accessory organ is developed from the postero-inferior 

 portion of the olfactory involution, which is of doubtful signifi- 

 cance. It first appears in larvae of about 12.5 mm. in length 

 (J. Fig. 18, PI. IX) as a small diverticulum of epiblastic cells 

 below the sensory cells of the olfactory epithelium. This diver- 

 ticulum gradually increases in size and apparently becomes con- 

 stricted off from the nasal chambers, though, possibly, it retains 

 some connection with them. (Fig. 20, PI. IX). In this stage 

 the posterior wall is much thicker than the anterior, and espe- 

 cially thickened in the median line. In transverse sections of 

 larvae 26 mm. long, the diverticulum is seen dividing into two 

 lateral portions by means of a median constriction. Anteriorly, 

 the two portions remain connected, but posteriorly they are 

 separated and form two blind sacs. From this stage of 

 development I have detected no change during larval life. 

 In the adult, however, this simple rudiment has become greatly 

 enlarged and very much more complex, forming apparently a 

 large median gland, having many follicles which are lined by 

 low columnar epithelial cells. This gland is contained within 

 the cartilaginous nasal capsule at its postero-inferior portion, 

 and is situated below the olfactory lobes (Fig. 22, PI. X). It 

 is best seen in transverse sections, which show that its main 

 mass is placed between and below the olfactory lobes. I have 

 not been able to detect any opening of this apparent gland into 

 the chamber of the olfactory organ or into the naso-palatal 

 canal, though such communication not improbably exists. Ap- 

 parently all of the follicles are connected together. 



Just what this organ represents it is diflficult to decide, 

 though to all appearances it is the representative of the organ 

 of Jacobson. Gotte has described the formation of this organ 

 in Bombinator (Unke, p. 654) as a diverticulum from the nasal 

 pit, which becomes connected with a gland formed from the 

 epithelium of the stomodaeum. In Petromyzon the oral epi- 

 thelium has no share in the formation of the organ in question, 

 nor does the latter at any time have any communication with 

 the mouth. If this organ is to be identified with that of Jacob- 



