MONOGRAPH OF THE EXISTING CRINOIDS. 449 



nuclei, lying on one of the cells which stood out from the rest on account of the 

 somewhat larger nucleus lying near the cell wall from which the calcareous particle 

 appeared to have been secreted by a sort of cuticular extrusion. As the calcareous 

 particle becomes larger progressively more and more cells take part in the deposition 

 of lime about its edges as well as over its surface. 



In the middle of the anterior portion of the embryonal body between the 

 second and third ciliated bands lie the rudiments of the columnals. They are 

 closely placed one behind the other in a single slightly curved row the concave 

 side of which is directed ventrally. 



The foremost element of the row, the terminal stem plate, lies near the base of 

 the vestibnlar invagination. In some cases, if the curve of the series of columnals 

 is especially strong, the plane of this plate is almost parallel to the major axis, 

 though usually it makes an acute angle with it. The most posterior stem plate 

 lies within the circlet of the infrabasals, displaced a little clorsally. 



At this stage, as well as at the following stages, the number of the columnals 

 is variable; very commonly it is 11, but there may be lo. If a considerable 

 number be present new columnals form always at the posterior end of the series 

 and are never intercalated. 



The form of the individual columnals is very variable. Seeliger gives three 

 figures of different stages. The first shows a minute calcareous crescent, of which 

 the concave side is directed ventrally. In the second the ends of the somewhat 

 larger crescent have become swollen and a broad straight-edged process has 

 appeared centrally. In the third each angle of this straight-edged process has 

 been produced into a radial spine, the swollen ends of the crescent as seen in the 

 last stage have become produced into radial spines, and the horns of the crescent 

 have become still further produced, so that the concavity now is almost semi- 

 circular. In this same series the more anterior columnals were markedly larger, 

 and had already extended so far ventrally that the concave side formed three- 

 quarters of the circumference of a circle. The radial processes on the convex side 

 had become more numerous and had begun to fork at the point. 



If a large number of columnals be present new ones are formed always at 

 the posterior end of the series, and new plates may appear through intercalation 

 between the last two, between the penultimate and the preceding, or between the 

 third and fourth from the last ; they probably never appear further anteriorly. 



In embryos of this age the formation of new plates between others already 

 existing is rendered possible by the existence of two or more layers of mesenchyme 

 cells between them. Usually the columnals lie so closely together that only a 

 single layer of mesenchyme cells lies between, which of necessity is occupied 

 with the increase of those plates. 



The terminal stem plate is the most extensive of all the calcareous structures 

 of the embryo, and is probably the first to appear, although this has not as yet 

 been determined with certainty. It resembles the calyx plates much more than 

 it does the other columnals, for it is in the shape of a rather extensive meshwork 

 which shows no trace of having been derived from a primitive crescent. 



