The intestine is covered vrith a visceral peritoneun (serosa) 

 composed of both smple and stratified squamous epithelia cells 

 in different places. Beneath this lies a relatively undifferen- 

 tiated muscular layer. The lumen is lined Tri.th pseudo-stratified 

 columnar epithelial cells, ivhich may or may not be ciliated, and 

 a variable number of secretory cells. A mucosa of connective 

 tissue forms a core for each of many ruga and is present at the 

 base of the folds also. Many small capillaries enter the rugae 

 from the Trails of the intestine. 



Even aconc the earliest migrant sea lampreys some constric- 

 tion of the lumen is evident (Figure Ul). As this cavity becomes 

 more severely constricted, the rugae become contorted and bent 

 upon themselves (Figure 1+2); a part of the evident reduction 

 in the lumen is due to croivding of the folds as the intestine 

 first shrinks. Following this, these structures become increas- 

 ingly truncated and aborted in shape as the diameter of the in- 

 testine declines in size. A fusion seemingly occurs at the bases 

 of the rugae and many disappear entirely (Figures U3-U6). 



In spavming sea lampreys the rugae are reduced, in cross sec- 

 tion, to blunted, misshapen knobs. Many disappear and some of 

 the remaining ones are broadly fused at their bases. At this, 

 the most reduced stage, the typhlosole is essentially devoid of 

 rugae, and ivith its large arterj^ and vein, occupies almost all 

 of the lumen of the intestine because it has regressed relatively 

 less per se than the remainder of the gut (Figures U7-U8). 



Aside from a gradual loss in the volume of tissue present, 

 no specific changes appear to occur in the serosal and muscular 

 layers. However, some very striking changes occur in the structure 

 of the epithelial lining of the intestine. This tissue is com- 

 posed of pseudo-stratified columnar epithelium in the se:cually 

 immature adult, A virtual retrogression of this layer occurs 

 along mth the gross changes previously recorded. Among migrants 

 in the earlier stages of sexual maturity, more and more simple 

 coluTJiar cells form the lining of the lumen apparently replacing 

 the pseudo-stratified coluBinar ones. As maturity advances and 

 the intestine becomes more and more constricted, the simple cell 

 tj'pe increasingly dominates the lining. In subsequent stages 

 the columnar cells become progressively shorter and broader. 

 This culminates, in specimens taken on the spaivning grovmds, in 

 a lining composed almost exclusively of simple, cuboidal cells. 



Although reduced in number and size, the blood vessels and 

 capillaries retain a highly fxinctional appearance. This is par- 

 ticularly so in the artery and vein of the tj^phlosole (Figures 

 U0-U8). Furthermore, verj-- fev/ degenerate or dead cells, either 

 free or attached, could be distinguished in any of the sections 

 studied; only apparently living, if somewhat altered, cells ap- 

 peared to be present, llo marked or progressive dehiscence of 



- 130 



