EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 415 



5. As seen io Antedon rosacea, according to Buky (Seeliger). Here the posterior element is 2' + 3, and divides as in 

 Synapta. 



6. As in Asterina gibbosa, according to Goto. Here the posterior ccelom (3) is said to be composed of a left element 

 from the left side and a right from the right, which fuse together. According to MacBeide, Asterina more closely resembles 

 Echinus (No. 3), the posterior ccelom coming from the left side only. 



7. As in Asterias pallida, according to Goto. Here the bilateral division is instituted ; and if Goto's work on this 

 form be confirmed, it gives us one of the most symmetrical types. 



The diagrams are based upon the structural facts as narrated by the various authorities, though there is no evidence 

 at present that any of them would agree with the interpretations of the changes as here given. 



This diagram clearly shows that closely allied species or groups may diverge widely in their ontogenetic stages, not 

 only externally and in such processes as segmentation and gastrulation, but in the methods of production of segments of 

 the mesoblast. 



Conclusion. 



We are thus led to the conclusion that the third body-cavities of the bilateral 

 ancestor have very early in the history of the Echinoderms fused together, and that the 

 only trace of their division is seen in such embryonic stages as the larva of Cribrella 

 and the primary separation of the posterior cavities in Asterina and Asterias, which 

 soon meet and fuse posteriorly. The median mesentery of Holothurians as well as the 

 circular transverse mesentery of other groups appears to be the line of junction between 

 right lateral ccelom and the two posterior cceloms. 



In concluding, I must take this opportunity of expressing my sense of gratitude 

 to my friend Prof. M'Intosh, at whose suggestion I commenced this work several years 

 ago, and to whom I owe much help and encouragement in its prosecution. I must also 

 express a sense of indebtedness to Dr Noel Paton and the Committee of the Royal 

 College of Physicians Laboratory for the facilities which have been freely placed in my 

 hands for the last two years. 



Summary. 

 The principal points dealt with may be tabulated somewhat as follows : — 

 Embryonic Period. 



1. Segmentation very variable, but always culminating in a solid morula of equal cells. 



2. A process of multicellular egression reduces the morula to a blastula, upon which 

 gastrulation takes place. 



3. The archenteron becomes filled with hypenchyme and blastopore closes. 



4. Archenteron divides into mesenteron, anterior ccelom, and posterior ccelom. 

 Embryo set free, with uniform coat of cilia. 



Larval Period. 



1. Stage A. — Similar to embryo. Anterior ccelom grows laterally into two lateral 

 cceloms. 



2. Stage B. — Posterior ccelom becomes differentiated into left and right. Larva 

 elongates and anterior coelom fills pre-oral lobe. Central ccelom developed from 

 posterior wall of anterior ccelom. 



