31 



This layer of protoplasm is extremely thin over the 

 greater part of the yolk, and is tightly stretched over the 

 protuberance fonned by the oil-globule. It is not, how- 

 ever, uniformly thin, but in one place has a disk-shaped 

 thickening. This thick disk of protoplasm (germinal disk) 

 is concave towards the yolk and convex on its 02:)posite 

 side, and is fitted like a saucer against one side of the 

 yolk. Its position is such that its center is upon the 

 equator of the yolk, so that in looking at the egg from 

 above one sees the edge of the germinal disk. Outside the 

 disk the layer of enveloping jDrotoplasm is so thin that it 

 cannot be easily seen except by the use of reagents. So 

 long as the yolk is within this enveloping layer of proto- 

 plasm it is entirely transparent and colorless. If the en- 

 veloping layer be ruptured so that the yolk passes out 

 and comes into contact with the water, it becomes instantly 

 opaque and of a milk-white color. 



The foregoing description applies to the egg after it has 

 l)een some time in the water. As the egg leaves the 

 female the egg membranes are not separated from the yolk 

 by a water-hlled space, but are everywhere in close contact 

 with the layer of protojjlasm which invests the yolk. 

 When the egg is placed in water, the water passes rapidly 

 through the egg membranes and accumulates between 

 them and the yolk. In this way the membrane becomes 

 gradually separated from the yolk by a water-lilled space 

 — " the breathing space." 



By this passage of water through the membranes they 

 become tightly stretched and tense, so that an egg which 

 at first feels under the finger like a piece of soft putty, 

 becomes hard to the touch by the absorption of water, and 

 feels like a shot. This "filling" of the egg takes about 

 two hours. 



The foregoing account of the structure of the egg is 

 sufficient to an understanding; of the mechanical 

 ments that it presents. 



