304 ME. J. E. DFEEDEN ON 



the gastrula, and consequently of the endoderm, to iuvagination, 

 are a misinterpretation of the appearances. 



In the course of development a primitive mouth or blastopore 

 appears to be always formed, and in some instances becomes 

 closed ; but in any case the permanent mouth and stomodaeum 

 are considered to arise by a later infolding of the ectoderm. 



At the conclusion of the delamination in Metridium McMurrich 

 (1891, p. 308) observed a slight depression at the posterior pole 

 of the larva, which soon after breaks through, a communication 

 of the interior cavity with the exterior being thus established. 

 His latest stages of this species did not permit of his 

 ascertaining the manner of formation of the stomodseum. A 

 primary mouth, with a later, invagination of the two-layered 

 wall to form the stomodseum, is figured by Kowalewsky for 

 Actinia (sp. ?) and Ceriantlius, and by Jourdau for Actinia 

 equina and apparently for the coral SalanophyUia (1881, 

 ' Embryological Monographs,' pis. xi.-xiii.). 



In Aurelia, Gotte found that after the pLinula settled down 

 an ectodermic invagination is formed, its lower end breaking 

 through, thus establishing a communication of the interior with 

 the exterior and constituting the oesophagus. First two and 

 then four diverticula or gastric pouches of the general cavity 

 are formed and arranged around the oesophagus, and a reflection 

 of the lining of the latter also appears tj take place. 



I believe it will ultimately be found that the early stages in 

 the development of Lehrunia will be better understood by a 

 comparison with what occurs in the Scyphomeduste than by what 

 happens in the Actinozoa. 



In Lebrmiia, we are confronted with the facts that a stomo- 

 dseal-like funnel already exists in association with a considerable 

 mesenterial development, and that up to the time of liberal iou 

 of the larvae no oral aperture is present, but that this is formed 

 shortly after extrusion. 



Can we then, in face of what is already known in Scyphozoan 

 embryology, assume that the first mentioned structure is an 

 endodermal-lined archenteron, the blastopore of which has be- 

 come and is still closed, and that the later invagination is the 

 one corresponding with that which gives rise to the mouth and 

 ectodermal stomodaeum in all other known Scyphozoan larvae? 



Until the study of earlier stages can settle the question 



