. 784 proceedings of the academy of [dec, 



Histogenesis of Marginal Organs. 



A. — In the larva. — The similarity in the appearance of the teu- 

 tacle-rudimeut in polyp and in gonosome make it desirable to 

 describe both in the same connection. The account of the origin 

 of the tentacles in the polyp was therefore reserved for this section. 

 At first the larval tentacle is merely a small round knob, externally, 

 and internally it is made up of a core of two or three eudodermal 

 cells. When the tentacles make their appearance the body wall of 

 the polyp is made up of the double layer of cells, the ectoderm and 

 endoderm, separated by the thin supporting lamella of mesogloea. 

 These three layers are pushed out somewhat in the growth of the 

 tentacle, the region of greatest activity being the eudodermal 

 layer, where the core of the tentacle is formed by a rapid out- 

 growth of the cells of the body wall, accompanied by multiplication 

 of these same cells. After some weeks the cavity of the coelen- 

 teron becomes drawn out in a diverticulum in the direction of the 

 axis of the tentacle, so that the upper part of the gastric cavity 

 becomes stellate in cross section. PI. XXXII, fig, 15, shows this 

 condition in a five -months' -old polyp. This cavity does not reach 

 out into the tentacle itself in any of the specimens which I studied, 

 but may do so before metamorphosis takes place. During the 

 whole of larval life, the tentacle is made up of a core of eudoder- 

 mal cells in a single row, as is the case in hydra. Fig. 1 1 shows 

 the first pair of tentacles only developed, and the cell-layers are 

 seen as described. The endoderm cells are filled with a loose pro- 

 toplasmic mass (PL XXXIV, fig. 24, End.) and the nucleus is 

 conspicuous. The condition which is seen in an adult tentacle 

 with several cells of endoderm surrounding the central cavity (fig. 

 23) is easily derivable from the larval condition by longitudinal 

 fission of the eudodermal cells, repeated until a cross-section of a 

 tentacle cuts several cells (PI. XXXIV, fig. 25). 



-S. — In the Adult. — The regularity with which the tentacles and 

 sense-organs make their appearance in the adult, as previously 

 described, makes it possible to locate with comparative certainty the 

 beginning of one of these organs upon the bell-margin. PI. 

 XXXIV, fig. 24, is from a section of a medusa, cut horizontally 

 at the point of origin of one of the tentacles. The figure shows 

 the aspect at the level of the tentacle, somewhat above the velum. 



