EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 393 



elements show progressive development — 1/2, 2/3 being very marked, 3/4 and 4/5 

 slightly less formed, and 5/1 only just arising. In a similar manner, the oral coelom 

 element of 1/2 is the best developed (fig. 102), 2/3 is next (fig. 102), 3/4 is at an earlier 

 stage (not evident from appearance in section, but 2/3 runs throughout four or more 

 sections, whilst 3/4 only extends through about 3 or 4), 4/5 is absent at this stage, and 5 

 or 1 is only just arising (fig. 101). 



The hydroccele is now a ring, formed by an approximation of its oral and aboral 

 walls in the centre, and a breaking down of the same. This process has been described 

 in other asterids, and it does not appear necessary to figure sections illustrating the 

 point. The mesenteron has commenced to push out a process orally towards the 

 surface through the lumen of the hydroccele ring (fig. 102). but only after the lumen 

 has been formed ; it does not at this stage reach the epiblast. The epigastric 

 ccelom has become of more definitive shape, arching over the mesenteron aborally, 

 and forming a definite circular mesentery with the hypogastric ccelom (figs. 100 

 to 104). 



By the close of this stage the young starfish may be said to be definitely formed. 

 The organs that arise ab initio in stages E and F, such as the perihsemal elements and 

 the oral ccelom, all appear as axially symmetric organs, and the elements of these two 

 arise in a regular cyclical succession, agreeing with that of the hydroccelic and hypogastric 

 radii. Up to the end of stage F there is no appearance of the skeleton, the mesen- 

 chyme is still undifferentiated, and the epiblast shows no special developments. The 

 histology of the various organs which have been considered is very little altered from 

 that in stage D or earlier. Evidently the mesenteron has increased enormously in 

 thickness of wall, and certain of the ccelomic elements, such as the dorsal sac and the 

 epigastric coelom, have their walls distinctly thinned ; but the hydroccele has thick walls, 

 especially in the tube-feet, and the cells of the hydroccele are always more regularly 

 arranged than those of the other parts. 



In the Adolescent Period, commencing with stage G, we have to deal with the 

 completion of the perihsemal and oral cceloms, the formation of the ovoid gland, the 

 skeletal and genital systems, and the further development of the alimentary system. 

 It appears more desirable to pause here than to give an incomplete account of such an 

 important part of the development, especially as the account here given is more or less 

 complete in itself. 



General Considerations. 



Comparisons with other types. — After an extensive search amongst the copious 

 literature of the Echinodermata, I have failed to find any authentic instance of a 

 development at all resembling that of Cribrella. The total equal segmentation with 

 production of a typical blastula, followed by complete invagination to form a normal 

 gastrula, has been described in so many pelagic types, and also in the demersal types, 



