60 



is due to the strongly developed organs of generation lying within it. If these organs, as 

 is the case in other seasons than in the summer, are less developed, then neither will the 

 arms in full grown specimens exhibit such a remarkable enlargement; although they are 

 always at the base considerably thinner than they are further out from it. 



d. Development of the ambulacral skeleton. 



To examine the development of the ambulacral skeleton, it is only necessary to 

 isolate from the disc a recently formed arm-shoot, and to put it in a solution of potass, which 

 will quickly make all the organic parts transparent. On examining the arm-shoot under 

 the microscope with sufficient magnifying power (see Tab. VI, fig. 13, 14, 15) the calcareous 

 particles which are to form the ambulacral skeleton will then be distinctly seen deve- 

 loping themselves in the interior. As the formation of the various joints or metamera, of 

 which the arm is composed, always proceeds from the base of the terminal knob-like en- 

 largement of the arm, the youngest or earliest foundations of the vertebrae will always be 

 found nearest to the same, and the nearer to the base of the arms the more developed 

 they will be found. Thus in one and the same preparation, we have a whole series of suc- 

 cessive stages of development before us. Every vertebra is formed of numerous reticular 

 ramified hyaline calcareous staves, which take their issue from 4 different points or centres 

 answering to the single calcareous plates of which each vertebra is originally composed. 

 These points of ossification of the vertebrae are situated somewhat nearer to the ventral 

 than to the dorsal side of the arm, and they are arranged in pairs slightly varying in height, 

 and regularly alternating with each other. The pair situated nearest to the medial line, and 

 in a more dorsal position (fig. 14 a), forms the foundation for the ambulacral plates; that 

 more remote from the middle, and nearer the ventral side (ad), represents the adambulacral 

 plates. The former pair appears first, and afterwards successively the other. Originally 

 these points of ossification lie widely separated from each other, and only during the further 

 development and ramification of the calcareous particles approach each other little by little, 

 so as at last to become united partly by suture, partly by muscular ligaments. 



The manner in which the ramification of these calcareous parts composing the verte- 

 bras proceeds, may be traced very distinctly in a recently formed arm-shoot. First of all 

 there appears only a little angular calcareous granule (tig. 17, 18) which then shoots out a 

 certain number (usually 6) radiating processes (fig. 19) situated in the same plane. Each of 

 these processes divides itself then at the extremity, or shoots out in 2 diverging branches 

 (fig. 20) which are again subdivided, and at last partially coalesce with those adjacent, and 

 thereby, together with the original processes, form the boundaries of a single ring of oblong 

 apertures (fig. 21). By a dichotomic subdivision of the outermost processes continued in 

 the same manner, there is little by little formed a thin calcareous disc (fig. 22) perforated 

 with numerous holes ranged with tolerable regularity in concentric order. This disc begins 



