316 GILBERT C. BOURNE. 



by the mouth ; and, finally, it is supposed that these chambers 

 are the equivalents of the paired archenteric diverticula seeu 

 iu the embryo of Amphioxus, outgrowths which are eventually 

 nipped off to form the mesoblastic somites, the walls of which 

 constitute the mesoblast, the cavities the coelom. If the facts 

 adduced in support of this theory are not numerous enough to 

 warrant our giving unqualified consent to it^ there is at least a 

 great deal to be said in its favour. What is important to the 

 present purpose is, that if it be accepted as a probability, and 

 if further it be admitted as a general statement that, throughout 

 the Triploblastica this is the origin of the coelom, then by far 

 the greater part of what we understand by mesoblast in 

 the Triploblastica is homologous, not with the supporting 

 lamina, the Stiitzlamelle, of the Coelenterata, but with the 

 endoderm lining the cavities of the entocceles and exocoeles. 



If we seek for an explanation of the supporting lamina in its 

 origin we do not get a very satisfactory answer. Kowalevsky 

 (25) describes the development of a jelly-like interstitial tissue 

 between the cells of the inner layer of the thickened ectoderm 

 of the larvae of certain Alcyonarians. The inner cells eventu- 

 ally lose their primitive shape, become star shaped or spindle 

 shaped, and are separated from one another by an interstitial, 

 jelly-like substance. The outer ectoderm cells form a plaster 

 epithelium, which bounds the external surface of the animal. 

 In this case there is no doubt that the interstitial tissue, usually 

 called the mesoderm of the adult, is derived from the epiblast. 

 We have not so exact an account of the development of the 

 supporting lamina in any other group of the Coelenterata. 

 Fol (10) describes the appearance of a clear transparent jelly 

 between the two primary layers in Geryonia, but is unable to 

 state which layer it is derived from. Claus (2) is no more 

 explicit on the same subject in his work on Chary bdoea 

 marsupialis. Metschnikoff (' Studien fiber die Entwicklung 

 der Medusen und Siphonophoren') speaks of a similar jelly- 

 like substance making its appearance, but he does not say 

 how. Chun gives no further account of the origin of the jelly- 

 like substance in Ctenophora ; but, according to a recent paper 



