ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 897 



in the adult Medusa. Instead of the ordinary method — metagenesis, or 

 alternation of generations — direct methods of development, termed 

 by the author hypogenesis, may take place, by arrest of metamorphosis 

 at different stages. 



Of mere varieties in the development the following are the 

 chief: — Firstly, the gastrula may cease development before the 

 primitive planula cavity is obliterated by complete invagination 

 of the entodermal cells into the ectoderm ; the remains of the 

 planular cavity become the disk of a Medusa, while tentacles spring 

 from the lips of the gastrula mouth. Secondly, larvae of the 

 Scyphistoma-stage may develope several circles of tentacles, forming a 

 strobiloid growth, in which, however, the rings or metamerte preserve 

 their polype-like character, and do not take on that of Medusse 

 (Ephyruke), as in the true Strobila ; or the tentacles may branch, each 

 giving off two or three branches, or this branching may be confined 

 to the eight principal (viz. the four primary and four secondary) 

 tentacles, which each thus become triple. Thirdly, in the Strobila- 

 stage the ordinary production of from ten to twenty medusoid 

 heads, or " Ephyrulse," is very frequently reduced to that of one 

 such head, for the support of which the basal segment sometimes 

 becomes merely a pedicel. Fourthly, the " Ephyrula " or free, freshly- 

 budded stage of the Medusa, may either develope divided lobes and 

 sense-bulbs only on the four primary marginal lobes of the disk, the 

 secondary lobes bearing only trifurcate tentacles (a variety termed 

 Ephyrula connedens), or (E. spjhinx), the anterior, i. e. mouth-part, 

 may have the typical Medusa characters, while the hinder part has 

 the body of the Scyphistoma-polype persisting — a condition which 

 may be carried so far as to make the larva a sessile Medusa, as in 

 E. pedunculata, or a sessile polypoid Medusa, termed E. tesseroides. 

 The hypogenesis or direct development is effected by the arrest, as in 

 the first-mentioned variety, of development at the gastrula-stage, the 

 contents of the planular cavity becoming the gelatinous disk-tissue ; 

 the mouth of the gastrula broadens, the body flattens ; a groove de- 

 velopes round the mouth, and becomes the cavity of the umbrella ; 

 from the edge outside it, tentacular and ocular lobes sprout out. 

 The edges of the mouth grow downwards, the convex body flattens ; 

 the result is an Ephyrula-disk, which thus dispenses with the 

 Scyphistoma- and Strobila -stages. It remains still to be seen whether 

 these apparently startling variations, produced, as they avowedly are, 

 imder the operation of artificial conditions, occur with any frequency 

 in the natural state of things ; and further, whether the author's 

 deductions as to the morphology of the normally developed parts, 

 made from such varietal phenomena, can be considered to possess 

 the value of absolute fact. 



"Mouth-arms" of the EMzostomidse.* — 0. Hamann, attracted 

 by the question of whether these appendages are developed 

 from the gastric filaments (which would ascribe to them an endo- 

 dermal origin), or whether they are ectodermal in origin, has been 



* .Jenaisch. Zeitschr. f. Naturwiss., xv. (1881) pp. 24.3-85 (3 pis.). 

 Ser. 2.— Vol. I. 3 



