244 EMBRYOLOGY. 



and are thus incompletely separated. In the oldest stage the nerve mass is in 

 contact with the anterior surface at one point only, and its double character 

 does not appear until it divides to surround the stomodaeum. 



Apical Sense Organ: — An apical sense organ may exist in this anterior 

 depression from which the cerebral gangha arise. One is present in Mrjzomenia 

 banyulensis according to Pruvot, but in the present series of embryos there are, 

 with one possible exception, no especially developed apical cells nor tuft of ciha. 

 The exception is the oldest embryo where a small band of cells extends in the 

 mid line from the ganglionic enlargement bordering upon the stomodaeum to 

 the surface where it ends in a slight pit. The material is excellently preserved 

 yet it is not possible to detect apical cells, and ciUa have never been seen at 

 this point or anywhere else on the body. 



The Terminal Ring: — In Pruvot's figures of Myzomenia there appears a 

 circular group of relatively large cells surrounding the posterior end of the body. 

 This organ, somewhat resembhng an annelid telotroch, is ciliated and a diffuse 

 tuft of ciha projects from the enclosed, terminal depression. In some of the 

 oldest embryos of Halomenia the same structure, minus the ciha, holds a corre- 

 sponding position (Plate 13, fig. 11). The cells are comparatively large yolk- 

 laden elements (Plate 14, fig. 1, 2), resembling small test cells, and are arranged 

 about a saucer-shaped depression. At first they form two rows as in Myzomenia, 

 but in the oldest embryo in my possession the ring-hke arrangement has become 

 lost, the cell boundaries have seemingly disappeared and I have not been able 

 to detect any nuclei that may with certainty belong to these cells. The depres- 

 sion is likewise lacking and the yolk granules merely form a confused mass 

 (Plate 13, fig. 9) at the posterior end of the body. In this same section the 

 ectoderm adjacent to the "telotroch" appears to be passing beneath the yolk 

 granules, leaving them upon the exterior, but the absence of definite cell 

 boundaries renders this somewhat uncertain. Appearances suggest that the 

 terminal ring is a larval organ that, Uke the anneUd telotroch, is cast off. 



The fate of the cells within the terminal ring is uncertain. In early stages 

 (Plate 14, fig. 2) the depression is composed of yolk-bearing elements similar 

 to those of the ring itself though of smaller size. At a considerably later stage 

 (Plate 14, fig. 3) the cells in the corresponding position are relatively small, 

 without yolk granules and with indistinct boundaries, and it is reasonable to 

 conclude that they are the progeny of the cells originally included within the 

 terminal ring. Anteriorly they are continuous with the ganglionic cord extend- 

 ing along the ventral side of the body. In the oldest stage the nerve cord comes 



