THYMUS-LTKE STRUCTURES IN AMMOCOETES 143 



ther, but produced a tongue-like process which projects into the 

 pharyngeal cavity (similar to the tongue-like non-ciliated epi- 

 theUal process represented in text figure 2). 



The non-ciliated epithelium which has invaded the ciliated 

 epithelium of the epipharyngeal ridge begins to show histogenetic 

 actiWties in a larva between 20 and 30 mm. in length. The 

 nuclei of the cells of these areas, or placodes, wander out into the 

 underlying connective tissues and are transformed into lympho- 

 cyte-like cells. A study of the histogenetic processes in these 

 areas in various developmental stages leads to the conclusion 

 that these areas represent specialized regions of the branchial 

 epithelium which are suggestive of primitive thymus structures. 



From the foregoing description it is evident that there are in 

 all seven placodes. The seventh and the first are smaller than 

 the remaining ones but they take part in the histogenetic proc- 

 esses and are therefore to be considered true functional placodes. 

 The placodes increase in size with the growth of the larva. 

 In the mature larva, however, they show a depletion of cells. 



When the larva undergoes metamorphosis the whole structural 

 arrangement of the branchial-region is altered. In the single 

 specimen of a transforming larva of my collection, the adult 

 arrangement has been attained, so I am unable to describe the 

 nature of this process. In this transforming specimen I have 

 also been unable to find any remains of the epithelial placodes of 

 the larva. Serial sections of the branchial region of an adult 

 lamprey have also been examined but with negative results. 

 It is evident that an involution of the placodal organ has taken 

 place as one would expect of a thymus. This involution began 

 in the maturing larva and was completed in the early stages of 

 metamorphosis . 



HISTOGENESIS 



Early development of the pharyngeal wall in Petromyzon marinus 

 unicolor, Lampetra wilderi 



The search for a thymus in the lamprey larva has revealed an 

 unusual accumulation of lymphocytes in the lateral walls of the 

 branchial cavity. These accumulations were first observed by 



