CYCLOSTOMATA. 91 



similar structure is placed behind the last, and two similar 

 structures in front of the first persistent pouch. This pouch is 

 situated in the same vertical line as the auditory sack (au.v), 

 and would appear therefore to be the hyo-branchial cleft ; and 

 this identification is confirmed by the fact of two head cavities 

 being present in front of it At the front end of the branchial 

 region of the mesenteron is placed a thickened ridge of tissue, 



pn 



FIG. 43. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA 

 OF PETROMYZON. 



The larva had been hatched three days, and was 4 '8 mm. in length. The optic 

 and auditory vesicles are supposed to be seen through the tissues. The letter tv 

 pointing to the base of the velum is where Scott believes the hyomandibular cleft to be 

 situated. 



c.h. cerebral hemisphere; th. optic thalamus; in, infundibulum; pn. pineal gland ; 

 mb. mid-brain; cl>. cerebellum; md. medulla oblongata ; au.v. auditory vesicle; op. 

 optic vesicle ; ol. olfactory pit ; m. mouth; br.c. branchial pouches; th. thyroid invo- 

 lution ; v.ao. ventral aorta ; ht. ventricle of heart ; ch. notochord. 



which, on the opening of the passage between the stomodseum 

 and the mesenteron, forms a partial septum between the two, 

 and is known as the velum (fig. 43, tv). 



According to Scott (No. 87) a hyomandibular pouch forming the eighth 

 pouch is formed in front of the pouch already defined as the hyobranchial. 

 It disappears early and does not acquire gill folds 1 . The tissue forming the 



1 Scott informs me that he has been unable to find the hyomandibular pouch 

 in larvae larger than 4-8 mm. My material of the stages when it should be present is 

 somewhat scanty, but I have as yet, very likely owing to the imperfection of my 

 material, been unable to find Scott's hyomandibular pouch either in my sections or 

 surface-views. Huxley describes this pouch as present in the form of a cleft in later 

 stages; I have failed to find his cleft also. The vessel interpreted below as the 

 branchial artery of the mandibular arch was only imperfectly investigated by me, and 

 I was not sure of my interpretations about it. Scott however informs me by letter that 

 it is undoubtedly present. 



