MATERIALS FOR. A MONOGRAPH OF THE ASCONS. 575 



different individuals of the same species, and in the different 

 classes of spicules in the same individual. The embryo of 

 contorta shows the latter point very well. On the third day 

 the numerous spicules fall into two distinct classes: (1) 

 those which have the rays more or less equal in length, and 

 meeting at more or less equal angles ; (2) those which have 

 two larger rays placed in the same straight line, or nearly so, 

 while the third ray may be only slightly smaller than the other 

 two, or it may be very much smaller, or finally it may be 

 absent altogether, in which case the whole spicule is simply 

 spindle-shaped, and is a true monaxon spicule. It is evident 

 that the spicules of the second class are the young forms of the 

 large monaxons which characterise this species, and that these 

 spicules are secondary monaxons derived from two rays of the 

 primitive triradiate. 



In the embryo of contorta the rays seem in all cases to 

 unite when about 4 or 5 /x in length, or even earlier. The 

 spicules of the first class, the true triradiates, remain non- 

 crystalline for a much longer period, and only light up between 

 prisms when the rays have reached a length of 16 or 17 /*• 

 The spicules of the second class, on the other hand, the future 

 monaxons, become crystalline much earlier. I have not found 

 any which remained perfectly dark, while those with rays over 

 8 /x in length (i. e. with the monaxon shaft over 16 fx in length) 

 light up with the greatest brilliancy. 



In cerebrum also two classes of spicules are early distinct, 

 plainly corresponding to the two classes just described in 

 contorta. Those of the first class, the regular triradiates, are 

 relatively more abundant. In one embryo examined (A) they 

 become crystalline when the rays measure on the average 

 about 13 — 15 ju; in another (B) this event takes place earlier, 

 when the rays are about 10 or 12 ju in length. The spicules 

 of the second class can be recognised as the young forms of the 

 horn-like spicules peculiar to this species. In the adult, though 

 variable, they have typically two thick rays, placed nearly in a 

 straight line at their bases, but curving like horns towards the 

 tips, while the third ray, which lies in a different plane, is 



