246 EMBRYOLOGY. 



of the trunk, that is bounded by a dehcate though none the less definite cuticular 

 sheath. It has the appearance of a developing spine, scale or plate of calcareous 

 material, but with one specimen only it is not possible to form a definite opinion. 

 It is very evident that the oldest larva of Myzomenia as figured by Pruvot 

 with its many scale-like plates is much more highly developed than any embryo 

 in the present collection. 



Stomodaeum : — As previously noted a few of the cells on the ventral side of 

 the body, at the posterior border of the test, rest against a shallow depression 

 (Plate 14, fig. 7, 8) in relatively early stages, and extend for a considerable dis- 

 tance into the interior of the embryo. They thus occupy identically the same 

 position as the future stomodaeum, and it therefore becomes practically certain 

 that the depression is the first indication of the blastopore and that the slender 

 cells are stomodeal elements. In the next stage (Plate 14, fig. 3) in the present 

 collection the yolk-laden inid-gut, communicating with the exterior, is directly 

 in contact with the test cells anteriorly, but posteriorly it connects with the yolk 

 free stomodeal cells which extend for a considerable distance within the body. 

 The stomodaeum is thus a semitubular structure largely confined to the pos- 

 terior side of the digestive tract. This same state of affairs continues in the 

 oldest stage (Plate 14, fig. 4). 



Mesoderm: — After the formation of the ectoderm the remaining cells form 

 a confused mass within the interior. Order, that is regularity of arrangement, 

 is not established until relatively late in development (Plate 14, fig. 3) when the 

 mid gut is distinctly outlined. Between the gut and the ectoderm are several 

 fairly large yolk-laden cells whose exact nature is open to question. Anteriorly, 

 between the test and the gut, several of these appear (Plate 14, fig. 3), and 

 in the latest stage represented (Plate 14, fig. 4) these appear in even greater 

 abundance. They ultimately may become incorporated with the mid-gut, or 

 in part at least they may form the ectoderm of the anterior end of the body as 

 the test diminishes in size, or in whole or in part they may be mesoderm. In 

 the posterior end of the body similar cells appear, of somewhat smaller size, that 

 I am inclined to consider true mesoderm. Unfortunately the question cannot 

 now be decided. 



Endoderm: — The endoderm, as indicated in the preceding paragraph, 

 does not become clearly differentiated until relatively late in development, 

 judging from the material in hand. At the time that the stomadaeum com- 

 municates \vith the succeeding section of the digestive tract the mid-gut is 

 clearly defined as a bhnd sac filling practically all of the interior of the body. 



