EXPERIMENTS WITH FROG'S EGGS. 79 



has tried to account for the gastrulation process as the out- 

 come of the accumulation of certain waste products in the blas- 

 toccel, hut since the development of the gastrula may take place 

 when no blastoctel is present, this explanation of the mechanics 

 of gastrulation does not appeal to me as a probable one. 



REMOVAL OF THE ROOF OF THE BLASTOCCEL. 



It has been suggested by Rhumbler that the process of gas- 

 trulation may be due to the accumulation of waste products, 

 carbon dioxide for instance, in the blastoccel fluid. The pres- 

 ence of such a substance would bring about changes in the 

 surface tension on one side of the yolk cells, which, by causing 

 them to change shape, is imagined to bring about the intu ruing 

 of the cells. It seemed to me that this view might be tested 

 by emptying the blastoccel of its fluid before gastrulation had 

 occurred. If the inturning still took place the result would show 

 that the process need not be connected with the blastoccel fluid, 

 or with substances that have become dissolved in it. 



There was also another question that I wished to examine by 

 means of the same experiment. The formation of the large 

 blastoccel space takes place at the time when the embryo-forming 

 materials, that come from the upper hemisphere, are moving out- 

 wards and downwards at the sides of the blastoccel, and the 

 question arises whether this movement is connected with the de- 

 velopment of the blastoccel space. Finally there is still a third 

 question involved, namely, whether the movement of the mate- 

 rial is due to the downward pressure of the cells themselves of 

 the roof of the blastocoel. 



In the first set of experiments the roof of the blastoccel was 

 opened, and injured by plunging a needle into it. Despite the 

 operation the process of gastrulation still took place in most 

 cases, and a normal embryo developed. In consequence of the 

 operation, as sections show, a large part of the fluid of the blas- 

 toccel is set free, although a small part of it may remain. When 

 the operation is carried out at an early stage the cavity may de- 

 velop later and suffice to bring about the gastrulation, if it were 

 really due to this factor. In order to meet this possible objection, 

 I operated on t\vo other sets of eggs, one at the time when the 



