-45- 



In relation to the gill arches, etc. (Drawn from the living 

 animal under a magnification of 50 dlam) , 



Fig. 5 b . Transverse section through the head of a 

 f PetromyzorK^arva^ anterior to the thyroid, through the point 

 where (on the rig?it side) the ciliated groove turns from the 

 dorsal towards the ventral side of the pharynx. The section cuts 

 exactly through the end of the loop on the right side but some- 

 what posterior to this point on the left side, (ivlag, 240 diaci. ) 



Fig. 5 c. Transverse section a short distance poste- 

 rior to Fig. 5b , to show how the dorsal branches of the cili- 

 ated grooves approach each other, on the roof of the pharynx, 

 as they pasB towards the posterior; and the same for the ventral 

 branches on the floor of the pharynx. (Mag. 240 dlam.) 



Fig. 5 d . Transverse section posterior to Fip;. 5 c 

 and just anterior to the gland. Shov/s the dorsal ciliated 

 grooves separated only by a ridge of cells which a short distance 

 further towards the posterior becomes the ciliated dorsal ridge. 

 The ventral grooves also are seen nearer together. (Mag. 240 

 dlam. ) 



Fig. 5 e. Transverse section through the anterior end 

 of the thyroid. Shows the still nearer approach of the two ven- 

 tral grooves. The cover cells are flattened out Into a thin 

 membrane, y/hlle the gland cells are very long, are splitting off 

 from thsir nucleated ends the cells to form the ciliated layer 

 and begin to show an arrangement into the four cell-groups. 



