ECHINOCYAMUS PUSILLUS. 



39 



common pseiidopodial matrix is able to thrust out branches, in which 

 the calcification takes place of such spines as are of less importance to 

 the larva and variable in position. There is but little doubt that the 

 calcareous deposits in other Echinoderms originate in the same way as 

 above described, and when I take into consideration the mode of for- 

 mation and growth of bone in vertebrated animals, it seems to me 

 very credible that the ossifying ground-substance is not formed in an 

 intercellular substance, but is originated by direct conversion of the 

 plasm of the osteoblasts. 



VIII. 



The development of the Pluteus. 



With regard to the Pluteus in Echinoeyamus, there are three sub- 

 sequent stages to be discerned, which are distinguished by a different 

 number of centres of calcification and by a gradual increase in the num- 

 ber of rods and arm-like appendages. 



The first stage of the Pluteus is characterized by possessing only 

 two centres of calcification, and consequently two rods and two larval 

 arms, viz. the posterior ventral ones; besides, during the latter period of 

 this stage, two anterior ventral arms begin to grow out. The changes 

 which the larva undergoes during this stage, are in short as follows. The 

 Gastrula curves slightly towards the ventral surface, which becomes con- 

 cave, and its ectodermic cells flatten except at the animal pole, where 

 they remain high and cylindrical, PL III, fig. 37 — 39. Then the two 

 first arms of the larva begin to arise. At first they appear as two small 

 protuberances, arranged symmetrically one on each side and slightly in 

 front of the blastopore, and in them a latticed rod is visible which has 

 protruded from the calcareous star on each side of the larva. It almost 

 seems as if the protuberances were caused just by the growth of the 

 calcareous rods towards the ectoderm, forcing this to bulge out. The 

 protuberances rapidly increase so as to form the long narrow posterior 

 arms characteristic of this pluteus stage, PL V, fig. 82 — 85. 



Forty-eight hours after the fecundation, the larva has reached a 

 total length of about 0,2 mm. and the arms have attained about half 

 this length, PL III., fig. 40 — 41. At this time the ciliated band is also 



