THE AXIS OF THE EMBRYO. 



149 



mining the relation of the early cleavage planes to the median 

 plane of the embryo. Dean found " in the early stages of seg- 

 mentation that the cleavage planes occurred in the normal way 

 when the position of the ^gg was reversed." This is true 

 not only for inverted eggs, but for eggs placed in any posi- 

 tion whatsoever. It seems to follow that gravity can have 

 no directing influence on the cleavage. In order to ascertain 

 whether there is any constant relation of the embryonic axis to 

 either of the first two cleavage planes, eggs were fixed in given 

 position by weighting, as before mentioned, and a sketch of the 

 early grooves was carefully made in each case. These grooves 

 are easily identified for a long time in the lower hemisphere of 

 the Qgg, even as late, in some cases, as the early stages of 

 gastrulation. As the sketches made at successive intervals 

 showed no movement of the o.^^ during all this time, it seemed 

 probable that the position of the ^g^ remained practically 

 unchanged up to the time when the median plane of the 

 embryo was ascertainable. In some cases accidental markings 

 on the surface of the Qgg remained in a fixed position until the 

 embryo was well defined. 



In 1893 Morgan 1 experimented with the eggs of teleosts. 

 The pelagic eggs of Ctenolabrus and Scrranus were selected. 

 Morgan says : " My results show that there is no relation 

 whatsoever between the cleavage planes of the &gg and the 

 median plane of the adult body. I have definite records of 

 twenty-two eggs carefully marked. The results, expressed as 

 nearly as possible, show for Ctenolabrus the plane of first 

 cleavage agrees approximately with the median plane of the 

 body 'vi\ jive cases, the plane of second cleavage with the median 

 plane in ten cases. The median plane of the adult body lay 

 between the first and second cleavage planes in two cases. 

 For Serranus the first cleavage agreed in one case, the second 

 agreed in two cases, and neither in two cases." 



Two methods were employed. In one case the egg was 

 watched continuously, and the median plane of the embryo 

 corresponded to a plane midway between the first and second 

 cleavage planes. 



1 An. Aiiz., viii. Jahrg., p. 803. 



