462 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



skeletal- forming mesenchyme cells reach the ectodermal vestibular invagination, 

 there being no connective tissue layer between. 



In addition to the rounded and star-shaped cells elongated strongly refractive 

 cells stand out prominently, which appear to be muscle fibers in process of for- 

 mation. They appear to have no definite arrangement but run in various directions. 

 In later stages they appear to become still more elongated fibers, mostly passing 

 quite through the loose connective tissue layer, which bear on the side a plasmatic 

 cell body containing the nucleus. 



In addition to the muscular fibrillse there occur rather prominent, mostly 

 undulating, smooth fibers travei-sing the connective tissue which show neither a 

 nucleus nor a superposed cell body. These are probabty elastic fibers, though 

 possibly only simple connective tissue fibrillae. 



The fact that larvae of this age are able to alter their form to a considerable 

 degree shows the existence of functional muscle fibers. 



By far the greater part of the mesenchjTiie cells in the posterior portion of 

 the animal, which later becomes the calyx, and in the central portion of the anterior 

 section, are occupied in the formation of the calcareous plates of the calyx and 

 column. 



In the last stage described all of the calcareous elements were laid down except 

 a few columnals which appear later intercalated between those at the calical end 

 of the column. During a large part of the free-swimming period there ensues 

 only an enlargement of the individual plates and insignificant alterations in their 

 relative positions. 



Their form and size vary exceedingly in different individuals. In many they 

 have developed only slightly, while in other larvae the columnals have become con- 

 siderably enlarged and are no longer crescentic, but have assumed the form of 

 closed rings, though this usually first takes place in the fixed stage. 



The orals and basals of the left side (1 and 2), which at first were situated 

 more anteriorly than those of the right, have moved posteriorly, so that they are 

 now on the same plane as these ; but the ventral orals and basals always lie farther 

 anteriorly than the dorsal, and the planes in which the orals and basals lie are 

 approximately at right angles to the water vascular ring. 



The infrabasals lie at the same height as the basals but more centrally, so 

 that they are covered by them. They are disposed in a horseshoe open ventrally 

 and toward the left. 



The calcareous rudiments of the columnals have moved farther from each 

 other, and the terminal stem plate thus approaches the base of the attachment pit 

 to which it lies nearly or quite parallel. 



The relation between the mesenchjTne cells and the plates which they form 

 can best be determined on longitudinal sections through the anterior part of the 

 larva. Such sections show the invariable existence of two layers of strongly 

 rounded mesenchyme cells between each two columnals. Beyond these, outside 

 of the compass of the calcareous plates, similar cells lie closely appressed to each 

 other, which later will take part in the enlargement of the plates and in the 

 formation of their peripheral processes. 



