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opened and partly removed, by means of a needle, at different 

 stages in the cleavage process. Nevertheless, the materials that 

 form the embryonic ring continued to push down at the sides of 

 the egg and a normal embryo was produced. The experiment 

 is open to the obvious criticism that after the removal of the roof, 

 or a part of it, the opening soon closes again ; but if the down- 

 growth were really due to pressure of the blastoccel fluid or even 

 to the pushing of the cells of the roof on each other, the dis- 

 organization of the process, that would probably follow, when a 

 part of the roof is removed, must be so great, one would think, 

 that it is unlikely the downgrowth could subsequently take place 

 normally. Still the experiment is inconclusive and does not 

 settle the question. 



The two most conspicuous changes that take place during the 

 segmentation and gastrulation stages are the development of 

 the enormous segmentation cavity and its disappearance during 

 gastrulation. We may look upon \\\e purpose of the segmentation 

 cavity as a space into which the lower cells may push when the 

 upper cells pass over them. While the presence of the segmen- 

 tation cavity may facilitate this process, we gain no insight into 

 the origin of the cavity or of its disappearance by a considera- 

 tion of its purpose. Its formation seems to be due largely, as I 

 have already said, to a secretion of fluid from the surrounding 

 cells ; its disappearance is more difficult to explain. The ques- 

 tion resolves itself into these alternatives --is the blastocoel fluid 

 absorbed or is it forced out of the egg in the later stages of gas- 

 trulation ? Since the egg does not decrease in volume or very 

 little, as the yolk surrounds and finally obliterates the segmenta- 

 tion cavity, the fluid would seem to be absorbed. It is true, we 

 might assume that the fluid is squeezed out of the egg and water 

 from the outside absorbed at the same rate, but this assumption 

 only complicates the question, and in the end amounts to the 

 same thing. Admitting that the fluid is absorbed, can this ex- 

 plain the inward migration of the yolk. I think not, because 

 in the first place the process of ingrowth takes place around the 

 sides of the blastocnel cavity and around the lips of the blastopore 

 and not throughout the entire floor of the segmentation cavity, and 

 in the second place there is nothing in the changes that take place 



