THYMUS-LIKE STRUCTURES IN AMMOCOETES 137 



ORGANOGENESIS OF THE PHARYNX IN THE AMMOCOETES 



A complete detailed description of the development of the 

 branchial region in the lamprey larva is apparently not to be found 

 in the literature. Separate structures and the condition in a 

 single or in a limited number of developmental stages, however, 

 have been described by various investigators. These descrip- 

 tions have been accurate with the exception of minor details, 

 but having been limited to a single stage in most cases they do 

 not include the changes which occur with the growth of the 

 larva. There are consequently contradictory statements in 

 the litcT-ature on the pharynx of the ammocoetes and especially 

 in the part dealing with the ciliated grooves and bands. Fur- 

 ther, the formation of structures which I interpret as primitive 

 thymus placodes is closely linked with the changes which occur 

 in the ciliated bands in the branchial lining. 



The following descriptions are based entirely upon my own 

 material : 



The transformations in the early larvae are very rapid so that 

 in 6 and 7 mm. larvae gill lamellae have formed on the branchial 

 septa, the pouches open to the outside, and the epithelium is rep- 

 resented by more than one layer. A system of ciliated epithelial 

 grooves and bands are present in this stage of development of the 

 pharynx. They form a connected system which may be looked 

 upon as beginning in two rather deep diverticula in the caudal 

 walls of the first pair of gill pouches. From each diverticulum 

 two ciliated grooves originate, one passing ventro-caudally, the 

 other dorso-caudally. These will be designated the ventral and 

 doisal grooves respectively. 



The two ventral grooves converge in a caudal direction as 

 far as the third pair of gill pouches where they come to lie 

 close together and parallel to each other near the median line. 

 Between the two grooves is a median ridge of non-ciliated epi- 

 thelium w^hich disappears in the fourth pouch where the two cili- 

 ated grooves fuse to form a single one. A tubular divericulum 

 passes from the ventral groove into the thyreoid a short distance 

 caudad of the point where the grooves fuse. A second diverti- 



