MONOGRAPH OF THE EXISTING CBINOIDS. 



409 



In Isometra vivipara the same thing occurs, but in this case the eggs have pre- 

 viously been fertilized, this process taking place in the ovary itself. 



EGO CASE. 



The whole surface of an egg extracted from the ovary just before the normal 

 time of extrusion (that is. after the formation of the swelling indicating the posi- 

 tion of the subsequent rupture of the pinnule wall) is covered with a dark mesh- 

 work, the open spaces in which are closely set and regularly spaced circular areas. 

 In Antedon mediterranean the eggs of which at this stage are about 0..3 mm. in 

 diameter, these circular areas have a diameter of approximately 0.022 mm., and 

 there are from 16 to 20 of them about any equator. 



This meshwork lies neither on the outer surface of tlie transparent egg case, 

 which is entirely smooth, nor in its substance. The egg case bears on its inner 

 surface, immediately contiguous to the egg cell, peglike processes with broadly 

 rounded tips which project into the substance of the egg cell, and it is these pegs 

 seen in optical section which form the circular areas in the meshwork when the egg 

 is viewed from the exterior, the dark portion of the meshwoi'k representing the 

 spaces between the processes where the dark yolk comes nearer the outer surface. 

 The yolk carries on its surface pit-like depressions, in which the peg-like processes 

 of the egg case fit. 



After the egg has been exposed for a short time to the action of sea water the 

 pegs on the inner surface of the egg case become gradually lower and lower, and 

 the cavities between them shallower and shallower, until finally the inner surface of 

 the egg case smooths itself entirely out and the egg case becomes a spherical shell, 

 everywhere uniformly thick. Simultaneously the surface of the yolk becomes 

 smoother and smoother, until at last it shows no trace whatever of the pits into 

 which the pegs originally projected. 



A section of the egg case at an intermediate stage shows the border between 

 it and the egg cell as a crenulate line which gradually straightens out so that 

 finally both the inner and the outer borders form concentric circles. It is to be 

 noted that as the pegs on the inner surface of the egg case disappear the thickness 

 of the case between the pegs increases. 



The smoothing out of the inner surface of the egg case appears to begin 

 very soon after the eggs are laid in sea water, for the intermediate stage just de- 

 scribed is reached at the furthest in 10 minutes. 



If the egg is so treated so that the yolk contracts and shrinks away from the 

 inner surface of the egg case the former almost entirely obliterates the pittings 

 on its surface, while the peg-like processes on the inner surface of the latter are 

 not in any way affected. 



Reichensperger was able to comfirm the preceding observations of Ludwig, 

 and he found further that as the peg-like processes on the inner surface of the 

 egg case decreased in size the outer surface developed very much smaller fine- 

 pointed processes, so that the fully matured egg is covered with fine prickles. 



