THE ORIENTATION OF THE FEOG's EGG. 381 



aggerated degree. Very few eggs at this early stage in the 

 development show such a marked diflPerence of the two halves. 

 In some eggs of this stage the segmentation seems equally 

 advanced on both sides, but these are rather the exception than 

 the rule. I have never found a single case in any stage 

 where the pigmented side was in any way in advance of the 

 lighter side. On the contrary, the reverse is true in almost 

 every egg towards the close of segmentation, and in most 

 cases a superficial glance will reveal the fact plainly. 



Sections of the egg parallel to the third furrow show the 

 cells smaller over one hemisphere than over the opposite, and 

 prove at least that there is an unequal development of two sides 

 of the egg, and that the difference which exists between them 

 is no superficial one. 



A careful examination of the early blastopore stages of the 

 egg with reference to the pigment and to the unequal develop- 

 ment shows conclusively that the blastopore makes its 

 first appearance on the less pigmented and further 

 developed side of the egg, and, moreover, at a short 

 distance only from the group of large cells around 

 the lower pole. 



I have examined over a hundred eggs at this stage, and my 

 best observations have been made on eggs in which the de- 

 velopment of the yolk-cells, as compared with the rest of the 

 egg, was retarded, so that the outlines and size of the cells, as 

 well as the unequal development of the two sides, could be 

 plainly seen by surface views with a dissecting microscope. In 

 some eggs it was very easy to follow out the outline of the 

 yolk-cells around the lower pole after the formation of the 

 blastopore, though the development was often too far ad- 

 vanced to make this out satisfactorily. But wherever I could 

 follow out the cells it was plain that the region around the 

 blastopore was in advance of the opposite side. In most 

 cases it is difficult also to orient the egg as regards the pig- 

 ment after the appearance of the blastopore, though I had a 

 number of specimens where this could be done. Towards the 

 close of the segmentation period pigment rapidly forms over 



