DEVELOPMENT OF SPELERPES BILINEATUS 217 



the first plane and the median plane of the embryo. Furthermore, 

 his results show that the egg is not isotropic, as maintained by 

 Hertwig and others. 



The reason for this view may become clearer if we consider the 

 position of the entire embryo and its median plane in relation 

 to the first plane of cleavage. When the last two coincide, the 

 first plane of cleavage will separate the material of the two lateral 

 halves of the embryo. When the first plane of cleavage and the 

 median plane of the embryo lie at right angles to each other, it 

 follows, since the head of the embryo is located somewhere 

 between the upper pole of the egg and the equator, and since 

 the blastopore closes at or near the lower pole, that the embrj^o 

 develops entirely in one blastomere, excepting, possibly, a little 

 of the tail. In this connection, it must not be overlooked that 

 speaking generally, the material out of which a variable amount 

 of the posterior part of the embryo is formed lies in a half ring 

 around the equator and that the two meridians lying 90° or there- 

 abouts from the point of origin of the blastopore, which indicates 

 presumably the median plane of the embryo, are at the posterior 

 limits of the embryonic material; that is, these meridians where 

 they cross the equator, mark out the tail region of the future 

 embryo. Pricking experiments, notably those of Miss King, 

 show that this material moves nearly along these meridians, 

 towards the point of closure of the blastopore. i** In Spelerpes, 

 this material moves a little obliquely anteriorly (see page 224). 

 Even if the embryonic material lies higher up in the egg than I 

 have indicated, especially in earlier stages as Morgan maintains, 

 it would not affect this argument since this material is supposed 

 by him to move meridianally to the equator. 



If the first plane of cleavage and the median plane of the embryo 

 happen to coincide, as they usually do in the frog's egg, the second 

 plane of cleavage, which according to Roux separates anterior 

 and posterior, will coincide with the point of greatest extension 

 of the lateral lips, i.e., the point where the horns of the crescent 

 lie, at the moment that the blastopore becomes a half circle. 



1° See also below. 



