tc the color of the liver of the specimen exanined sis it was 

 obsei'ved that this character itss the most simply recorded 

 measure of the individual's degree of sexual maturity. The 

 validity of this association may be verified in the striking 

 correlation "between the two changes). 



A spawning run, as it progresses in time throU£^ migra- 

 tory and spavnriing periods, is composed of individuois in increas- 

 ingly advanced stages of sexual maturity. Among the earliest 

 migrants studied^, the average diameters of the intes Line varied 

 from 7.U to 10.8 millimeters . The average diameters decreased 

 thr-oughout the rim until among late arrivals maximum intestinal 

 diameters averaged U^l millimeters. Further reduction occurred 

 during the journey upriver and upon the spawning grounds. 'The 

 diameter cf the intestine of specimens collected while spaivning 

 never exceeded 2.5 millimeters s they were occasionally reduced 

 to a diameter of 1.5 millimeters. Rectal diameters were con- 

 sistent throughout in their progressive reduction, like those of 

 the intestine, although invariably somewhat lower in value. 



It is important to ncte that these gut diameter measure- 

 ments represent only a partial measure of the decrease in the 

 digestive capabilities of the intestines of the specimens studied. 

 An increasingly greater amount of potential digestive and ab- 

 sorptive surface of the intestine is lost with each small decrease 

 in diameter. The word "potential" is used advisedly since my 

 studies indicate that no feeding takes place during the time in 

 which this reduction is occurring. 



Cross-sections were prepared from the mid-portion of the 

 intestines of a series of sea lampreys to determine more pre- 

 cisely the changes in gross structure and histology which were 

 associated with its reduction in size (Figures Ii.0-I;8). This 

 material was collected from sexually immature, actively feedii'ig 

 adults, from migrants in var-ying stages of sexual maturity, and 

 from adults taken from their spasming redds. In all, intestinal 

 sections from 20 specimens taken before and during migration 

 ana diorin.g and after spaivning were examined. With the exception 

 of the sexually immature specimens, intestines were removed from 

 live material and preserr-'ed in F-A-A solution. Sections vrere 

 imbedded, cut at 10^, and stained in Harris ' Hematoxylin and 

 Eos in. 



The intestine of a sexually immature adult is circular or 

 ovoid in outline; its lumen is U-shaped. Into the lumen projects 

 e. typhlosole "vrfiich commences anteriorly on the dorsal side of 

 the intestine, describes a partial spiral, and terminates on the 

 T'entral side. The typhlosole does not usually extend more than 

 half wsy across the lumen. It carries both an artery and a largex' 

 vein. Many slender, closely-spaced longitudinal folds or rugae 

 arising from both the wall? of the intestine and from the tj'phlo- 

 sole likeiwise project into -ohe lumen (Figure UO), 



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