376 The American Naturalist. . [May, 



Then His' derivation of the the fourth ventricle from a 

 bending of the brain, is not supported by experiments upon 

 frogs and chicks in which, to be sure, a ventricle-like fold was 

 produced by actual bending ; but this was not in the position 

 of the fourth ventricle and was, moreover, the result of the 

 reaction of the living substances, not simply mass correlation. 



An ingenious research into the cause determining the 

 direction of the first cleavage plane in the egg and hence the 

 median plane of the adult frog, as it is thought, was subse- 

 quently published in full by Roux 8 in 1887. 



The method employed was simply to place eggs from the 

 oviduct upon glass, white pole downward and allow them to 

 adhere. Then sperm was added either by touching a cut 

 made in the jelly with a brush moisteneil by sperm or else by 

 introducing sperm through a capillary tube into one point of 

 the jelly. Examined at frequent intervals the appearance ot 

 the cleavage planes was noted and referred to marks made 

 upon the glass or paper under egg. 



When the eggs were fastened in the normal position and 

 were not eggs laid late in the season, when abnormalities are 

 frequent, the following results were obtained : 



Before fertilization but one axis of the embryo is indicated 

 in the egg : the main axis of egg, that from black to white 

 pole representing the ventro-dorsal axis of the actual embryo, 

 which the author believes has its neural folds formed upon 

 the white pole when held in a normal position. Yet this axis 

 corresponds to a cephalo-caudal direction of the embryo when 

 we consider the rearrangement of material that takes place 

 during gastrulation. Of the infinite planes passing through 

 this axis that one becomes the median plane of the frog which 

 is determined by the plane of copulation of the male and 

 female pronuclei. This plane of copulation in turn is not pre- 

 determined but may be put in any meridian by localized 

 fertilization. 



The side selected for entrance of sperm becomes the ventro- 



8 W. Roux: Beitrage zur Entwicklungs mechan.k ties Embryo. 



-.-■•■■'■• ■ " 



