DEYKLOPMENT OF CUCUMARIA ECHINATA. 



379 



The gastrula, uniformly covered with cilia, gradually elongates 

 along the future body-axis, and rotates actively around that axis, with 

 its apical pole directed forwards. The archenteron lengthens into a 

 long flattened vesicle more than half as long as the whole body, and 

 finally its posterior half bends and coils so that the whole archenteron 

 assumes the form of a sinistral spiral (Pl. V, figs. I and 2). At this stage 

 the following three parts arc distinguishable in the archenteron : — 



1. The antcriormost part, which is large, round and flattened 

 from right to left, making a right angle with the frontal plane, with 

 its hind end approaching the lefc side of the body (hy') 1 . 



2. The middle part which goes round transversely from left to 

 irght across the dorsal side and slightly bending anteriorly near its 

 middle (en'). 



3. The last part, which is in direct connection with the blasto- 

 pore, runs almost parallel to the body-axis, and has a very narrow 

 lumen (g f ). 



The first part is the future hydrocœle, the second part the 

 enterocœle, and the third part gives rise to the gut. This fully 

 developed gastrula stage is reached during the second day of develop- 

 ment. 



The next change which occurs in an old gastrula is the formation 

 of the stomoda:um. At about the middle of the ventral side, the 

 ectoderm begins to thicken inwards, and at last its inner surface comes 

 to touch the ventral edge of the anterior flattened part of the archen- 

 teron (Pl. V, fig. I, stm'). On the outer surface of this thickened 

 ectoderm, a little to the left of the sagittal plane, the stomodaeum 

 appears as a lungitudinal slit-like depression. 



1) The orientation as to right and left of the embryo can not be ascertained with any 

 degree of certainty from an examination of this stage alone. But in later stages, where the 

 midventral radi.il canal marks the midventrum of the body, the position of the stomodseum 

 is found to be on the left side, and, as seen in cross section of the body, at an angle of 

 about 30 from the midventral lîne (Pl. V, figs. 5 and 6, stm). The stomodrcum makes its 

 fisrt appearance in the very old gastruli, a little to the left side of the plane containing 

 the flattened part of the archenteron. 



