75 



vented the segmentation of the yolk around each nucleus. Whether 

 these conditions would influence the division of the nuclei themselves 

 is not apparent from these experiments, but that the nuclear activities 

 are influenced by them to a certain extent seems very probable in view 

 of some facts to be mentioned presently. 



The character of the segmentation is determined therefore by the 

 interaction of two forces: namely, first, the force determining the form 

 of the protoplasmic masses, and second, the amount, size and specific 

 gravity of the enclosed inert substances. If we knew these mechanical 

 conditions of weight, pressure and surface tension, etc., we could 

 predict the resulting type of segmentation as well as the form of the 

 future organs of the body. 



We can readily see how conditions such as these can influence 

 the resulting type of segmentation. Only a slight exaggeration of the 

 conditions in Limulus would be necessary to produce a typical mero- 

 blastic egg such as we find in vertebrates. Of course it has been gene- 

 rally recognized that the amount of yolk present in an e^^ would 

 have a great influence on its segmentation; but just how it acted has 

 not heretofore been clearly and experimentally demonstrated. 



The egg acts like a number of vesicles enclosed in a hollow sphere. 

 Fill the vesicles with some light expansive matter and in any position 

 they will remain nearly alike in shape, resembling the segmentation 

 spheres of a holoblastic egg ; but if in addition they contain an inert 

 substance whose weight exceeds the expansive force acting within, 

 we would have a condition like that just described in the e^^ oî Li- 

 mulus, and like what probably prevails in most meroblastic eggs: that 

 is, in whatever position we place the sphere, the contained masses 

 will always be clearly outlined and more or less protuberant on the 

 upper surface ; but these characters will become fainter on the sides 

 and finally disappear altogether beneath, owing to the pressure of the 

 overlying masses, etc. 



It is therefore obvious that the character of the segmentation is 

 not influenced so much by the amount of yolk, as is usually stated, as 

 by its specific gravity. This fact probably explains why some eggs 

 with a comparatively large amount of yolk are holoblastic, while others 

 with comparatively little yolk are meroblastic. 



If the yolk and protoplasm were of the same specific gravity, 

 both would probably be uniformly distributed throughout the ovum. 

 When the difierence in specific gravity between the two substances 

 increases, a time comes when one must occupy one side of the ovum 

 to the exclusion of the other. As the yolk becomes heavier, it seeks 

 the lower pole of the egg, and it becomes increasingly difficult for the 



