142 



BIOLOGICAL LECTURES. 



the opened ovary of the fish into a glass dish containing just 

 water enough to cover them. After becoming fixed to the 

 bottom they were fertihzed. The side of the Qg2, resting on 

 the glass was much flattened, and the fact that the blastodisk 

 retained its position at the free pole of the Qg^ when that pole 

 was directed upward, led to the statement that " the adhesion 

 of the yolk to the egg-membrane, as it rested on the disk area, 

 prevented rotation." 



In the experiments of '96 and '98 the eggs were allowed to 

 fasten themselves on pieces of glass which were inverted and 

 which rested on supports within a large dish of sea-water. Con- 

 sequently the cleavage and development had to be observed from 

 below, by looking up through the bottom of the glass dish. 



In this case the eggs were in their natural position, that is, 

 in the position in which they are usually deposited by the fish 

 when making her nest under a flat stone. 



It makes no difference, then, how the egg is attached, 

 whether on the roof of the nest, or on the side or on the floor; 

 the axis of the egg is ■Av^'A.ys, perpendicular to the plane of attach- 

 ment. The fish usually fixes its eggs so that they hang from 

 the surface above ; but if they are deposited in a piece of stove- 

 pipe, so as to cover the upper half of the concave inner surface, 

 the position of the axis of the egg will vary at all angles 

 between a vertical and horizontal position. 



If the eggs are dropped loosely into a glass dish containing 

 very little water, some of them will fall so as to become quickly 

 fixed to the bottom of the dish, while others may fall so that 

 the adhesive surface fails to come into contact with the glass, 

 and so soon loses its adhesive property, leaving the egg free to 

 roll. The axis of the fixed eggs will be vertical, but the free 

 pole will face upward instead of downward, as it does when 

 the Qgg is placed on the roof of the nest, or on the underside 

 of a glass plate. It is evident, therefore, that gravity has no 

 decisive influence in determining the direction of the egg axis. 

 In other words, the Qgg has a definite and constant position in 

 its membrane, the vegetative pole always lying at the centre of 

 the adhesive disk, while the animal pole and micropyle coincide 

 on the opposite side. 



