A MODIFICATION OF THE URODELE TESTIS. 159 



a point in development beyond which, under the now unfavorable 

 conditions, further progressive changes seem inhibited. Xor can 

 these cells remain in a resting condition until the advent of 

 spring. Their degeneration and resorption from the caudal 

 lobules of the testis follows, with the results already poimed out. 

 Strangely enough, the point in development at which the cells 

 are stricken varies with the species. There i-, apparently, no 

 one "critical period" common to the four species studied. The 

 fii! in that the germ cells suffering degeneration an- in 



each case tho>e most advanced in development 'od in the 



nio-t caudal lobules of the testis. In Plethodon glntinosus \\ 

 cells are spermatids. Spermatocytes in their Drouth period .ire 

 not alirctrd. Yet it is in this latter stage that the v^erm cell- of 

 Ciyrini'/)hilns begin degeneration. Rather curiously, tliose germ 

 cell- . if both Gyrinophtius and Plethodou which chain e to In- 

 tiaii-toniiiii^ from spermatogonia to spcrmatocyte^ n-main 

 un.ilh < ted. This transformation period, it will In- recalled, U 

 tint in \\hich the "boundary plane" degenerations always begin. 

 It U the cells of this stage of development which degenerate in 

 ]-'.H ry< i-<t and J'!, thodon cinerins to produce the caudal appendage. 

 The absence of such an appendage in numeroii> male- of these 

 i\\o spe< ies might be assumed to be due either to .m alienee of 

 cell- .it the critical stage, or to an increased ability of tin - celU 

 to Mir\i\e unfavorable conditions. That the appendage is 

 seldom al>M-nt in Gyrinophilns and Plethodon glutinosns \- e\ iilence 

 ot the constancy with which germ cells reaching more advai; I 



^-ta^e- degenerate. 



So f.tr as m.i\ be perceived, there is nothing inlu-rent in their 

 cainl.il po-ition in the testis which should cause -en-a cells to 

 dem -lu-raie. This region is apparently as well va>culan/ed as 

 an\ ])ortion of the testis; its germ cells in autumn show no 

 indication of interference with their development, bcin^ always 

 somewhat in ad\ ance of those more anteriorly Mtuated. Xor do 

 11 i elK of </// stages degenerate here. The primary sperma- 

 toiidiiia al\\a>> remain unaffected, and later restore the lobules of 

 the append. t^e through their proliferation. The degeneration, 

 in short, i- clearly correlated with the more advanced stage of 

 tlexelopment of the cells involved, only these cells being unable 

 to survive when conditions become unfavorable. Such un- 



