144 EXTERNAL FACTORS III. 8 



and the same must be said of the skeleton. Normally there are 

 two tri-radiate spicules, one to the right, the other to the left 

 of the gut and some little way from it. Without the needful 

 sulphate the spicules become placed near the gut, and may with 

 the growth of the latter be pushed towards the animal pole. 

 The number of spicules may also be diminished or increased 

 to one, three, or four, arranged in a circle round the gut. On 

 timely removal to sea-water, however, a secondary bilateral 

 symmetry may arise by two of these outgrowing the rest and 

 stimulating the development of the typical arms of the Pluteus. 

 It seems that a sulphate is present in the calcareous skeleton 

 of the Pluteus, as there is in that of the adult urchin. 



A ciliated circum- oral ring is formed, but is abnormal in its 

 position, at right angles instead of parallel to the long axis of 

 the body. The pigment which should be secreted by the 

 secondary mesenchyme cells (separated off from the inner end 

 of the archenteron) remains in abeyance, and the apical tuft of 

 cilia is hypertrophied. Other processes, however fertilization, 

 segmentation, and ciliary motion are independent of SO 4 . 



The development of eggs which are allowed to remain in 

 ordinary sea-water until the blastula stage is no better, whence 

 Herbst concludes that no SO 4 is taken up during segmentation. 

 During the early stages of gastrulation, however, they appear to 

 absorb a store of it for future needs, for gastrulae reared in 

 sea- water develop further in the SO 4 -free solution than do 

 those embryos which have been kept in it since fertilization. 

 SO 4 is equally necessary for the continued life of the Pluteus 

 and of the Bipinnaria larva of Asterias, and without it the rate 

 of regeneration of the head of Tubularia is retarded and the 

 number of tentacles reduced, until eventually a completely 

 tentacleless head is evolved. 



The necessary sulphate can be, to a certain extent, replaced 

 by a thio-sulphate. The addition, for instance, of -35% Na 2 S 2 O 

 to the SO 4 -f ree solution renders it possible for the larvae to reach 

 the Pluteus stage, though the arms are short, the skeleton small, 

 and the pigment reduced. The larvae die. 



Selenium and tellurium are both poisonous in an early 

 stage. 



