DEVELOPMENT OF SPELERPES BILINEATUS 215 



lowed to the time of the appearance of the blastopore, no constant 

 relation existed. In spite of the lack of a coincidence between 

 the first plane and the median plane of the embryo, several 

 authors, expecially Roux for the frog, maintain that a causal 

 relation really exists, but fails to manifest itself owing to the 

 presence of disturbing factors. I shall return to this question 

 a little later. 



The results of many observers who found that, in a large per- 

 centage of cases, the median plane of the embryo coincided 

 with the first plane of cleavage, point, in a general way, as Roux 

 maintains, to some sort of relation between the two. Nevertheless, 

 I believe with Schultze ('99) that whatever the relation existing, it is 

 equally certain that it is not a causal one. In Spelerpes, it is 

 certain that there is no definite relation existing between either 

 the first or second plane of cleavage and the median plane of the 

 embryo. My results appear to show, furthermore, that as far 

 as Spelerpes is concerned, eggs selected at random have not been 

 subject to any external influences which might make the first 

 plane of cleavage take a different position from what it would 

 have had, had such influences been absent. Roux ('03) has taken 

 the stand that frog's eggs, as obtained from the field, are not 

 normal, that is, they have not been kept from those conditions 

 which cause the first plane of cleavage to deviate from its proper 

 position and therefore, he has objected to the use of results ob- 

 tained on such eggs. It is well known that in nature frog eggs 

 when first deposited lie with their axes in all sorts of positions 

 relative to gravity. Only after some time do they become free 

 to rotate and thils bring their axes into a vertical position. Ac- 

 cording to Roux, eggs lying with their axes placed obliquely are 

 in abnormal positions. Thus, paradoxically enough, the eggs 

 although normally laid, are laid in abnormal positions. It has 

 been shown by Born that, when the egg's axis is placed obliquely 

 to gravity, the contents rotate. Roux believes that the entrance 

 point of the sperm determines the first plane of cleavage and the 

 median plane of the embryo. Now, since he fails to find an exact 

 agreement between the two, some explanation is necessary, which 

 is found in the fact that the copulation path of the sperm nucleus 



