44 Hjalmar Théel, 



may be minute ectoplasmic processes more or less crowded over the 

 free extremity of the cells, and subsequently these become either arran- 

 ged in groups or, as is the case in the bottle- shaped cells of the Plu- 

 teus and probably even in the collar-cells in the Porifera, they fuse so 

 as to form a ring or collar of clear plasm round the cilium. 



In this stage of development we also find inside the ectoderm 

 irregularly branched pigment-cells, Pi. IV, fig. 71 — 7.5, which contain, 

 besides an accumulation of coloured pigment-granules, a good many 

 small refracting globules and a large nucleus. Often the pigment- 

 cells intercommunicate by their branches. As the larva advances in 

 development, it gradually deepens in colour owing to the increasing 

 aggregation of pigment-molecules. Besides, filaments and cells with 

 amœboid processes connect the digestive tract with the body-wall of 

 the larva. 



Towards the end of this developmental stage, the left vaso-peri- 

 toneal vesicle has pushed its way to the surface of the dorsal median 

 line and passed into the water-pore. 



The second stage of Pluteus commences to develop at eight or ten 

 days after the fecundation, PL VI, 86 — 56", supposing that the process 

 has gone on in a normal way, and is indicated by the rise of two new 

 centres of calcification which originate one on each side of the larva in 

 the angle formed by the posterior ventral arms and the oral portion of 

 the body. By the agency of the mesenchyme cells a calcareous three- 

 armed star arises in each centre. The rods of the star pass, one 

 slightly forwards and the other two in an opposite direction. One of 

 the latter, which keeps on the ventral surface, remains short and ends 

 with a small slightly perforated plate, PL IV, fig. 65^ while the remai- 

 ning one runs dorsally, gives off branches and crosses the correspon- 

 ding rod from the deposit of the opposite side. From the centre of 

 each deposit a latticed rod takes its origin, PL VI, fig. 86 — 88 y, and 

 extends into the posterior dorsal arms of the larva which are just be- 

 ginning to grow out. 



Simultaneously, as the second pair of posterior larval arms pro- 

 trude, or sometimes slightly before, an ectodermic invagination takes 

 place on the left side of the larva, PI. VI, fig. 89 x^ presenting itself 

 at first as a small, well defined heap of cells. This is an indication 

 of the future sea-urchin. 



The third stage of Pluteus begins at about fourteen days after the 

 fecundation and is distinguished by obtaining a new unpaired centre 



