STRU'?TURE AVD DEVELOPMENT OF THE TKYROIt) GLAND 

 IN PETROWYZON. 



Inti'oduotion. 

 The material with which the following work was done 

 was of tv,'o, or possiblj' three, epecies. The first species was 

 Pt planerij the small lamprey that, in the adult condition, x-each- 

 es a length of about six inches only. This f-irslr"±&fc:r-«4i mate- 

 rial v/as collected by Dr. R. G, Harrison at Naples. It included 

 stages from the recently fertilized egg to the swimming larva 

 in which the yolk was no longer externally visible and in which 

 the mouth was separated from the pharynx only by a thin parti- 

 tion of cells. This oldest stage was killed fourteen days after 

 fertilization. , , - 



The aecond. io-t of material was obtained at Ithaca, 

 N. Y, , and probably includes larvae of both F» ,d_or8%tus. Wilder 

 (Lake lamprey) and P, branchialis (Brook lamprey), as both species 

 are knov.'n to spawn in the same nest and it is impossible to tell 

 to v;hich species the larvae belong. Part of this lot of mate- 

 rial was sent me through the courtesy of Prof. S, H. Gag© of 

 Cornell; the rest I myself obtained from one of the "nests" 

 in a stream at Ithaca, New York. 



