REGENERATION IN EGG AND EMBRYO 22 5 



quickly added a part of the missing side. This reply fails, however, 

 to meet Hertwig's description of the method of development of the 

 embryos. Later work, however, has put us in a position to give a 

 more satisfactory account of the differences between the results of 

 Roux and Hertwig. It seemed to me that the two kinds of embryos 

 might be due to the different positions of the eggs after the operation. 

 It had been shown by Schultze ('94) that if a normal egg in the two- 

 celled stage is turned upside down and held in that position two 

 embryos develop from the egg (Fig. 63, B, C, D). These embryos 

 are united in various ways, and arise presumably one from each of 

 the first two blastomeres. These results have been confirmed by 

 Wetzel, who examined more fully into the early development of the 

 twin embryos. He showed with much probability that the proto- 

 plasm rotates in each blastomere, so that in many cases the lighter 

 part flows, or starts to flow, toward the upper hemisphere of the egg. 

 In this way similar protoplasmic regions of the two blastomeres 

 may become separated, and under these circumstances each blasto- 

 mere gives rise to a whole embryo. A cross-section through one of 

 the segmentation stages of one of these eggs is shown in Fig. 63, A. 

 The smallest cells are found at the outer side of each half, and the 

 two segmentation cavities lie one in the upper region of each hemi- 

 sphere. Some of the different kinds of embryos that develop from 

 inverted eggs are shown in Fig. 63, B, C, D. They are united in Fig. 

 63, B, by their ventral surfaces, and in Fig. 63, C, C 1 , C 2 , by their 

 dorsal surfaces, and in Fig. 63, D, D 1 , at the sides. These differences 

 are probably accounted for by the different ways in which the proto- 

 plasm of the first two blastomeres rotated before the egg divided. 



A consideration of these results led me to carry out the following 

 experiment on eggs operated upon by Roux's method. After stick- 

 ing one of the first two blastomeres, some of the eggs were placed 

 so that the uninjured blastomere kept its normal position, i.e. with 

 the black hemisphere upward. Other eggs were turned, so that 

 more or less of the white hemisphere was upward. From the two 

 kinds of eggs two kinds of embryos were obtained. From those with 

 the black hemisphere upward the embryo was a half-embryo like that 

 described by Roux, while from the eggs with the white hemisphere 

 upward embryos developed that were in many respects whole embryos 

 of half size. 1 The explanation of this difference will be obvious from 

 what has been said. When the black hemisphere is uppermost the 

 contents of the uninjured blastomere remain as in the normal egg, 

 and a half-embryo results. When the white hemisphere is uppermost 

 the contents of the uninjured blastomere rotate, so that it generally 

 shifts its relation to the protoplasm in the other injured half, and a 



1 In one case a half-embryo resulted. 

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