210 BENJAMIN H. GRAVE. 



Cumingia cannot be kept in the laboratory indefinitely. It is 

 desirable to use them the day they are collected, although it is 

 sometimes possible to keep them for a day or two in moist sand. 

 They must be kept away from water if it is desirable to have 

 them retain their eggs or sperm. 



THE EGG. SIZE, STRUCTURE, MEMBRANES. 



The egg of Cumingia measures in extreme limits from .061 

 mm. to .065 mm. while the usual size is .062 or .063 mm. In 

 color it varies from a light gray to a distinct pink. The animal 

 pole of the egg is comparatively free from yolk and pigment so 

 that it appears light in color. Centrifuged eggs show a beautiful 

 banding due to the separation of materials of different specific 

 gravity. These materials consist of yolk, pigment, oil and clear 

 protoplasm. 



The polarity of the egg is fairly rigidly fixed as shown by the 

 fact that in centrifuged eggs the cleavage planes remain un- 

 changed although the pigment and so-called formative materials 

 may be driven to any part of the egg. This general problem 

 was investigated by Morgan in 1910 and the phase of it referred 

 to here was fully verified by myself in 1924. He was concerned 

 with the influence of the "organ-forming substances" upon 

 development. 



Surrounding the egg there is a jelly membrane about .03 mm. 

 in thickness or half the diameter of the egg. It is wholly invisible 

 under the microscope but readily demonstrable by the use of a 

 suspension of india ink in the water. One might suspect the 

 presence of this jelly membrane by the circumstance that the 

 eggs in a dish do not come into contact but are separated by a 

 little space. This tertiary membrane has apparently been over- 

 looked by investigators. There is also a distinct vitelline 

 membrane which undergoes little or no visible change after 

 fertilization. The membrane is not perceptibly lifted after the 

 penetration of the spermatozoon so it may be proper to say 

 that there is no fertilization membrane. This may account for 

 the fact that the Cumingia egg is at the mercy of the sperm and 

 polyspermy is common. At any rate it is desirable to learn 

 whether this susceptibility to polyspermy is du^ to absence of one 



