DEVELOPMENT OF SPELERPES BILINEATUS 211 



that it does not stay long enough for the jelly to become dried 

 out. Five or six eggs can usually be marked while waiting for 

 the stain to work through the jelly of the first. When this point 

 is reached the egg is carefully floated in a drop of water and re- 

 moved from the slide to permanent quarters. The membranes 

 soon swell, leaving a stained spot on the egg. Sometimes the 

 egg rotates within its membranes and new marks may be pro- 

 duced on other parts of the egg if sufficient stain remains in the 

 jelly. 



Eggs whose peri-vitelline fluid has been removed are usually 

 incapable of rotation for a time. Before its removal, the egg 

 rotates so quickly that the upper hemisphere is almost always 

 uppermost when the fluid is removed. The simplest way to in- 

 vert the egg in order to mark the lower hemisphere is to use a 

 perfectly dry second slide. The first slide with its egg is inverted 

 over the second and carefully lowered until the egg adheres well 

 to the second. As it is the drier, the egg sticks to it more strongly 

 than to the first, which is easily removed, leaving the inverted 

 egg behind. 



Owing to its inability to rotate, care should be taken to orient 

 the egg, animal pole up, after they are placed in permanent 

 quarters. Unless this is done the egg is subjected to the influ- 

 ences of gravity in abnormal directions, which, as is well known, 

 may produce abnormalities. 



The attempt to follow the marks into the interior of the egg 

 by means of sections has failed thus far, since the stain dissolves 

 out in alcohol. By breaking open the egg after preservation 

 but before placing it in alcohol a stained spot made on the dorsal 

 lip was found only a short distance within the egg. It is hoped 

 to be able to work out the movement of material within the egg 

 by means of eggs preserved in formalin. 



All drawings of marked eggs, except most of those made to 

 determine the relations existing between the plane of first cleav- 

 age and the median plane of the embryo, were made with a camera. 

 Since they are projections of a sphere upon a flat surface, it should 

 be borne in mind that a point on the surface 45° from the egg 

 axis for example, comes to lie approximately four-fifths of the 



