IV 



COELENTEI1ATA 



person develops into a medusoid, passing, in some cases at least, 

 through a hydroid stage in the course of its development. 



In the development of Geryonidae, a family of the Trachy- 

 medusae, it is usually stated that a spherical blastula is at first 

 formed, and then that the inner vesicular 

 portion of each cell of the blastula becomes 

 detached from the outer end, and that these 

 inner portions unite to form an endodermic 

 vesicle. In this way, it is said, the two- 

 layered condition is reached. 



This view is founded exclusively on 

 views of living segmenting eggs which, 

 owing to their spherical character, could not 



be orientated; and on the assumption that FIG. 44. EmbryoofaGeryonid 

 this mode of the formation of endoderm 

 and ectoderm actually occurs, it has been 

 regarded as typical delamination, and as 

 representing the primitive way in which a 

 two-layered condition was arrived at. 



Strong objections may be urged against this view. The 

 development of the Trachymedusae and Narcomedusae is greatly 

 modified as compared with that of the more normal Hydromedusae, 

 on account of their mode of life ; and further, when we consider 

 how easily mistakes can be made as to the nature of a process, unless 

 carefully orientated embryos are examined and cut into sections, we 

 must regard it as very questionable whether the kind of delamination 

 described in the Geryonidae does actually take place. It is possible 

 that in this family we have to do with a proliferation of the cells 

 forming the blastula wall, at one side of the blastula, but that the 

 area of proliferation is of considerably greater extent, relatively to 

 the whole surface of the blastula, than it is in the case of the blastulae 

 of ordinary Hydromedusae. If this pole were turned towards the 

 observer, he would receive the impression that he was looking at a 

 sphere, from the whole of whose circumference cells were being budded 

 inwards. 



( Carmarina fungiformis) in 

 which endoilerm cells are 

 being budded oil'. (After 

 Metschnikoff.) 



en//, enclodenn. 



II. SGYPHOZOA 



AURELIA 



If we now turn our attention to the great group of the Scypho/u;i, 

 we find that the development of the genus Aurelia has been fully 

 worked out, the latest accounts being given by Hem (1!)00) and 

 Friedemann (1902). These workers used a mixture of 100 parts 

 concentrated solution of corrosive sublimate, with 2% acetic acid, to 

 [reserve the larvae of Aurelia. 



This common jelly-fish swarms on both sides of the Atlantic. 

 As in all Scyphozoa the genital cells are produced from the endoderm 



