THE ORIENTATION OP THE FEOG's EGG. 387 



whether the remaining hemisphere would develop a perfect 

 half-sized embryo or half an embryo. Many eggs went to 

 pieces, both during and subsequent to the operation. Others 

 partially rounded up and continued to develop, but the 

 greater number of these died later. The few embryos that 

 formed the medullary folds were very imperfect^ but, as each 

 egg stuck had not been carefully followed during the stages of 

 segmentation and gastrulation, I hold these results to be 

 valueless. They show, however, I believe, the possibility of 

 carrying out the experiment successfully. 



In several eggs at the eight-celled stage one of the black cells 

 was killed by pricking, so that its contents ran out. Such 

 eggs developed, and in the blastular stage defects were found 

 in the black hemisphere. In other eggs one of the white cells 

 was stuck, and, later, defects were found either in the white 

 hemisphere or just within the edge of the dark area. In these 

 cases no record was kept of the position of the particular cell 

 removed ; hence the results are of little or no value, and I think 

 the same statement will apply to the similar experiments of 

 Roux. Now that it seems to be possible to recognise, even in 

 the eight-celled stage, the relationship between particular 

 blastomeres and definite portions of the later embryo, more 

 successful results ought to be obtained. 



The consistency of the yolk in the eggs of the two species is 

 different. That of R. temporaria is more fluid, and the egg 

 collapsed more easily than in the other case. Owing to this 

 difference the eggs of the unknown species were far more 

 favorable for experiment, and the following results were made 

 on these eggs. 



In order to determine the extent of overgrowth of the lower 

 pole by the blastopore a large number of experiments were 

 made by slightly sticking the white cells below the blastopore. 

 By using a very fine and sharp needle an exceedingly small 

 injury could be made, so that only a few small cells protruded 

 from the surface of the egg. These, however, gave a definite 

 landmark for orientation. The determination of the extent 

 of overgrowth by injury to the lower cells has a great ad van- 



