METAMOEPHOSIS OF ASTERINA ÜIBBOSA. 



237 



Cut 2. 



posterior portion of tlie anterior enteroccel, which about this 

 time begins to assume the form of the axial sinus. The mode 



of formation of the circular en- 

 teroccel and the central portion 

 of the perihsemal system is shown 

 in cut 2, where the broken lines 

 represent the union which is 

 secondarily established between 

 the cavities, and completes the 

 circular enterocœî. This definit- 

 ive separation of the circular 

 enteroccel [from the perihœmal 

 system takes place in stage G 

 (fig. 15). 



The above statements have been carefully tested and verified 

 in serial sections and 1 have no doubt of their veracity. It 

 must at the same time be j^ointed out that the circular entero- 

 ccel as well as the central portion of the perihsemal system is a 

 rather composite structure, for, while the Isrger part arises from 

 the secondary left posterior enteroccel, the part lying in the 

 madreporic interradius is formed from the anterior enteroccel. 



With regard to the remaining portion of the radial peri- 

 hsemal spaces in Asterina gibhosa, I find that they arise in 

 exactly the same way as in Ästerias jjaUula, i. e. as solid masses 

 of mesenchyme cells placed in pairs on either side of the 

 radial plane, directly on the oral side of the water- vascular tube 

 and between the successive pairs of tube-feet. These masses 

 afterwards acquire lumen and those on one side of the radial 

 plane growing out towards each other fuse, and thus give rise 

 to the definitive condition of the perihfemal system. I have 



