38 EMBRYOLOGY OF THE STARFISH. Part I. 



side, near the edge of the disk. The madreporic body itself would have been lost 

 in a similar manner, had it not been possible to track it by means of its connection 

 with the circular tube (PI. VI. Fig. 12) ; and, even then, it was only by the closest 

 attention, and at moments when the position of the young Starfish was especially 

 favorable for the inspection, that the opening of the madreporic body could be 

 distinguished from that of the surrounding limestone cells. 



Formation of the actinal Limestone Surface. — The actinal side of the disk is at first 

 a narrow flat band (PI. VI. Fig. 3), following the general outline of the rays. 

 This band increases in breadth, loses its convex outline, and soon reaches the 

 terminal tentacle, when the actinal band has assumed a pentagonal shape. Inside 

 of this small pentagon is situated the ambulacral system, entirely independent, as 

 yet, from the limestone deposit on the actinal surface, the whole rosette of ten- 

 tacles expanding and contracting, with perfect liberty, in every direction. This 

 freedom soon ceases ; the points of the limestone pentagon develope rapidly towards 

 the. centre of the disk, and soon reach the base of the radiating canal (PL VI. 

 Fig. 7). There they unite by bridging the intervening spaces, and form five tri- 

 angular openings, enclosing the tentacles, which are still at liberty, with the 

 exception of this band across the base of the radiating tubes (PL VI. Fig. 9). 

 The additions made to this deposit of limestone take place more rapidly near the 

 bridge, where additional limestone cells are sent out, enclosing at first the basal 

 pair of tentacles, but leaving the remaining five still unconfined. The next pair 

 is then imprisoned by a similar process, without, however, interfering with the 

 terminal tentacles. Finally, the last pair of tentacles is surrounded in a like man- 

 ner, and all the tentacles are now confined somewhat as we find them in the adult 

 (PL VI. Fig. 12 ; PL VII. Fig. 1). A row of limestone cells, extending along the median 

 line, separates the base of the suckers, while transverse bands join the larger cells of 

 adjoining spaces. It is plain that the transverse bands correspond to the ambulacral 

 plates of the adult, and that, in the earlier stages, the embryo Starfish has no trace 

 whatever of any interambulacral system. This mode of formation of the ambulacral 

 system may explain the absence of interambulacral plates in the Crinoids and Ophi- 

 urans. The deposit of limestone is not sufficiently transparent to allow a good view 

 of the radiating canal, or of the formation of the vesicles of the tentacles. 



Formation of the Spines of the young Starfish. — We have seen that, at the time of 

 the closing of the young Starfish, the abactinal region is already covered with regu- 

 lar rows of spines (PL VI. Fig. 4). These spines are, however, simple warts, slight 

 protuberances, in which limestone cells are formed, connecting with the general net- 

 work. The cells of these spines are arranged in regular tiers one above the other: 

 the younger cells, formed at the base, being always more numerous, and pushing up 

 the older ones. All the cells send off Y-shaped appendages, which unite, forming 



