The Sexual Conditions of Myzostonia glabrum (F. S. Leuckart). 311 



been negative. Cells, like those of iig. 6, are frequently met witb, 

 but it is not difficult to determine tbat sucb cells are male in 

 cbaracter — sperm-motber-eells of tbe first generation. 



It has been sbown in preceding pages , tbat tbe first eggs to 

 arise appear as modified cells of tbe peritoneum (figs. 1, 2, etc.), and 

 tbat tbeir ovarian cbaracters can invariably be made out in stages 

 (of young disc-forms) far smaller tban tbat of 1 ram, on wbicb in 

 bis description Wheeler lays stress (pag. 241). In tbe general part 

 (pag. 265) be is less inclined to empbasise tbe point, and leaves tbe 

 exact period, at wbicb tbe supposed transformation occurs, an open 

 question. 



We are told tbat tbe dorsicolous forms migrate to tbe disc at a 

 stage, wben tbey are somewbere about 1 mm*, and tbat eitber before 

 tbey do tbis or just aftervvards tbey become bermapbrodite. In a 

 case of tbis kind, wbere fresb material is not available, tbe only 

 course open to anotber observer, anxious to test tbe point, is to take 

 a series of dorsicolous forms, and compare tbem witb a series of 

 small forms from tbe disc 2. 



Wheeler looks upon tbis procedure as an »error of metbod 

 and one in common sense« and, none tbe less, tbe one lacking in 

 tbe latter quality is in reality tbe same as bis own! 



As to tbe metbod, ali tbe forms studied bad been killed in 

 exactly tbe same way; tbey were ali embedded etc. at tbe same 

 time, and were, in fact, from beginning to end, treated exactly alike. 

 As tbere is, so far as I can see, no evidence of sbrinkage in tbe 

 specimens, and as tbey were ali killed in tbe same way, it is not 

 too great an assumption tbat any sbrinkage was fairly even tbrougbout 

 tbe lot. If one bad sbrunk, tben, probably, tbe otbers bad been 

 diminisbed in a corresponding ratio. But allowing some sbrinkage ^ 

 — and undoubtedly one must — Wheeler must apply it to ali, and 



' It is nowhere stated whether the measurements are of lìving, preserved, 

 or embedded specimens. 



- This is really the method adopted by Wheeler, and his results differ 

 from mine, because probably his examples were not numerous enough, and 

 thiis he failed to obtain disc-specimens smaller, and dorsicolous forms larger 

 than 1 mm. 



3 Experiments recently made in another connection on the sbrinkage of 

 Elasmobranch embryos in various fluids and various strengtlis of alcohol bave 

 shown that by the time the specimens are in 90 X alcohol the percentage of 

 sbrinkage is practically a Constant quantìty for embryos under 37 mm. 



