THYMUS-LIKE STRUCTURES IN AMMOCOETES 131 



the thymus placodes in the fishes. While lymphocytes are present 

 in these situations the evidence brought out in connection with 

 them is not sufficient to establish their origin or the cause of their 

 presence in these places. 



A contribution on the ganoid thymus (Lepisosteus, Amia) 

 has been submitted by Ankarsvard and Hammar ('13). They 

 found the organ a purely endodermal, unsegmented structure in 

 a medial position in the dorso-caudal part of the epibranchial 

 region. ''It nevertheless has a segmented origin and the epi- 

 branchial unsegmented thymus structure represents a secondary 

 alteration from the branchial segments." In the older develop- 

 mental stages there is a rich infiltration of lymphocytes into the 

 sub-thymic and perivascular connective tissue which stands 

 out in striking contrast to the conditions in an earlier stage. The 

 authors discuss the question whether this condition represents an 

 immigration into the thymus or an emigration from it They 

 believe that the cells have migrated from the placode and repre- 

 sent the beginning of an accidental involution. A lobulization 

 of the organ does not occur. In the adult Lepisosteus the thymus 

 is strongly involuted. 



The nature of the thymus in the ganoids as described by 

 Ankarsvard and Hammar is so suggestive of the conditions I 

 have found in the ammocoetes that it appears to me to repre- 

 sent but a very small advance beyond a primitive form in the 

 phylogenetic development of the organ. The origin of the 

 lymphocytes which were supposed to originally migrate into 

 the epithelial anlage in the Lepisosteus does not appear to have 

 been especially determined by the authors. 



MATERIAL AND METHODS 

 i 



The material which is the basis for this work was gener- 

 ously supplied to me by Prof. Simon H. Gage. It consists of a 

 series of specimens ranging from the segmentation sphere up to 

 and including a transforming larva and the adult These speci- 

 mens undoubtedly represent two species, Petromyzon marinus 

 unicolor and Lampetra wilderi (the lake and brook lamprey 

 of central New York). 



