428 BULLETIN OF THE 



but really an optical effect produced by the differentiation of the 

 protoplasm. 



If it is not the nucleus of the first segmentation sphere which has 

 misled the author into denying the existence of a membrane, it must be 

 that he has only seen the germinative vesicle after the beginning of its 

 metamorphosis. If there were previously any chance for doubt, the 

 peculiar spindle shape which both nucleus and germinative vesicle 

 assume now proves sufficiently the morphological equivalency of the 

 two structures. 



Immediately upon contact with the fecundating element in the case 

 of Serpula the granules of the yolk are much increased, and are seen 

 undergoing a slow rotary motion ; this causes an opacity of the yolk 

 which renders the internal layer (germinative vesicle) almost invisible. 

 The latter, therefore, does not disappear, but is simply obscured. This 

 formation of granulations is not simply a mechanical alteration, but is 

 accompanied by a chemical process that eliminates from the yolk 

 certain secondary liquid and gaseous products which accumulate be- 

 tween the yolk and its membrane ; it is thus that the yolk becomes 

 somewhat contracted and the membrane much dilated. In consequence 

 of the formation of granulations the external layer of the yolk becomes 

 more dense, and therefore the internal layer (germinative vesicle), being 

 less dense, is obliged to ascend to the surface of the yolk, and thus one 

 pole of the egg becomes clearer than the opposite pole. When the 

 vesicle has reached the superior pole of the egg, the vitelline membrane 

 is resorbed and an aperture formed through which the vesicle escapes 

 in the form of two or three drops, — '' directive vesicles," ^ — which 

 remain between the membrane and the yolk. These directive vesicles 

 only serve to determine the point of departure and the direction of the 

 first cleavage furrow.* 



The latter is not, however, the first indication of segmentation. 

 After the formation of the directive vesicles the rotary motion of the 

 granules ceases, and they are gradually arranged in two groups between 

 which the plane of division is to pass. The granulations do not remain 

 quite fixed, but are disposed in rays which depart from the centre of 

 each group. Gradually this centre enlarges until it acquires the form 

 of a nucleus, so that the rays produced from the granulations no longer 

 depart from a point, but from a circle. As the furrow of segmentation 

 advances, the rays become more uncertain. 



* It is an error for Stossich to connect with this view the name of " the distin- 

 guished Miiller, father of embryology," instead of that of Friedrich Miiller. 



