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protoplasm to penetrate it, and finally a typical meroblastic egg is the 

 result. When this condition is reached, some provision must be made 

 to fix the egg in a constant position during development, or else to 

 allow the quick adjustment of the ovum within its envelope to a state 

 of stable equilibrium before the relative distribution of yolk and proto- 

 plasm within the egg itself can be disturbed. If such a disturbance 

 occurred frequently, it would be impossible for any two eggs to deve- 

 lope alike because in every egg the tension within that determines 

 form would be different at corresponding times and places. 



Most meroblastic eggs rotate freely within the egg membranes 

 on albuminoid cushions that equalize the pressure very beautifully. 

 The relative position of yolk and protoplasm to each other and to the 

 direction of gravity is fixed with great precision, although free to move 

 under unusual stresses. 



The size of the yolk particles has a curious effect on the embry- 

 onic tissues, that to my knowledge has not before been commented 

 upon. When they are large, the individual tissue cells are unable to 

 surround them and consequently the embryos of both Insects and Cru- 

 stacea, where these conditions prevail, are characterized by their clear, 

 protoplasmic composition. On the other hand in the Arachnids, Ga- 

 noids and Amphibia, the yolk particles are relatively small and not 

 apparently markedly different in specific gravity from the protoplasm. 

 Here each cell takes in large numbers of yolk particles giving the 

 embryonic tissues of these forms their characteristic coarsely granular 

 appearance. 



It is very desirable that we possess reliable data concerning the 

 specific gravity of yolk and protoplasm in various types of eggs. 



The effect of gravity in cell division is well shown by the follow- 

 ing facts : 



Eggs that have been fastened to the glass in the manner described 

 above are allowed to develop undisturbed for from five to ten days. 

 At the beginning of this period a well formed blastoderm is developed 

 over the upper surface of the egg. This surface then has a different 

 naked eye character from what it had before. It is whiter, smooth and 

 shining and under a low power one can distinguish the polygonal 

 outlines of the blastoderm cells, with a clear spot in the centre marking 

 the position of the nucleus. On turning the egg over no nuclei or cell 

 outlines can be seen on the under side. As the egg developes the 

 margin of the blastoderm forms in some cases a more and more distinct 

 equatorial welt, with infolded edges, which slowly grows over the 

 under side of the egg, leaving a gradually diminishing »yolk-plug«, 

 in exactly the same way as in Amphibia, Acipenser, etc. The whole 



