686 PATTERNS AND PROBLEMS OF DEVELOPMENT 



elude the possibility, suggested by Brachet, that a predetermined dorsi- 

 ventrality may be altered by the sperm. It is not necessary to assume 

 that such alteration must always result in coincidence of median plane 

 and plane of fertilization meridian. It may only bring about more or less 

 approximation; or, if the angle between the two is large, the sperm may 

 have little or no efTect. The possibility that a dorsiventral differential 

 may be initiated in the oocyte by the pattern of follicular circulation has 

 already been noted (p. 664); this possibility is a tempting one, but it re- 

 mains for the future to discover whether it is a reaUty. 



Some recent experiments on the eggs of R.fiisca are of particular inter- 

 est here. It is well known that after removal from the uterus unfertilized 

 amphibian eggs orient to gravity, with heavier basal pole down, as soon 

 as the swelling of the jelly and the appearance of the perivitelline fluid 

 permit rotation within the envelope. Some time after insemination an- 

 other partial rotation begins, amounting to some 15° in certain urodeles 

 and to 30° in R. fusca and some other anurans. This rotation displaces 

 the apical pole ventrally, the basal pole dorsally, in the future median 

 plane, and in R. fusca coincides in time with formation of the gray cres- 

 cent. 



According to studies by Ancel and Vintemberger (1933, i935). the use 

 of local marks shows that this ''rotation of fertilization" is not rotation 

 of the whole mass of the egg but a movement of the "pellicle," that is, 

 of the superficial cortex over the deeper cytoplasm. On the dorsal side, 

 movement toward the apical pole is most extensive in the future median 

 plane and carries with it part of the underlying pigment, the region at 

 the lower border of the pigmented hemisphere thus partially deprived of 

 pigment becoming the gray crescent. In its median region, this is approxi- 

 mately 30° wide, and its width decreases laterally as the movement be- 

 comes less extensive. These authors find, however, that cortical move- 

 ment is not only toward the apical pole but from lateral regions toward 

 the median plane on the dorsal side, so that the cortex or "pellicle" be- 

 comes thicker dorsally, thinner laterally. Evidently the future dorsal re- 

 gion differs in some way at this stage from lateral and ventral regions, 

 and there is a graded difference between its median and lateral regions. 



In further experiment dorsiventrality and median plane have been de- 

 termined experimentally in eggs of R. fusca by Ancel and Vintemberger 

 (1938). Eggs removed from the uterus to a slide without water, with 

 polar axis inclined 45° from the vertical and with basal pole upward, ad- 

 here to the slide by the jelly and are inseminated by application of sperm 



