OVUM. 



21 



undergoing a slight change of form, fixes itself 

 by the narrowest end, and acquires tentacles 

 like a Polype at the other, amounting for some 

 time to eight. In this condition it appears to 



Fig. 16. 



Development of Rfedvsce. ( From Sars, Steenstrup, 

 and Dalyell.) 



a, b, ciliated free moving embryo from the ovum ; 

 c, embryo attached bj T its pedicle ; d, its tentaciila 

 beginning to be formed; e, with four, /; with eight 

 tentaciila ; g, the fully developed polype, producing 

 other polypes by gemmation ; h, i, k,~ transverse di- 

 vision and development of Medusae from the polype 

 stock or strobila ; /, a pile consisting of four Me- 

 dusoids just about to separate ; m, , and lower 



lateral view of Medusae separated from the polype 

 stock; o, more advanced, natural size: p, r, (from 

 Dali/ell), p, a pile of medusa discs separating, and 

 new tentacula formed on the polype at the base ; r, 

 the same, with more of the discs separated ; the 

 strobila returning to its polype state arid budding 

 at the side. 



be capable of multiplying itself, or producing 

 other similar attached Polypes by gemmation 

 from its side or base, or from a running stolon 

 below it. The subsequent change of each of 

 these polypoids is remarkable. It has been 

 described by Sars and Dalyell as follows : 

 The body undergoing some elongation be- 

 comes partially divided by transverse grooves, 

 into a range or column of imperfect Medusae, 

 attached still to each other by their adjacent 

 surfaces, but presenting at their borders, in 

 various degrees of advancement, the division 

 into rays or lobes which belong to the Me- 

 dusa ; the upper or terminal one having deve- 

 loped upon it a set of radiated processes dis- 

 tinct from the tentacles of the Polype and 

 much longer than those of the rest. These 

 young Medusas are successively separated from 

 the stock by the deepening of the transverse 

 clefts between them. They then move about 

 as independent animals, and proceed in their 

 farther growth and development to sexual and 

 other completeness. These bodies, therefore, 

 are subject to two kinds of multiplication, 

 which are very different : by simple gemma- 

 tion a number or a colony of Strobilae may be 

 produced, and by transverse fission and deve- 

 lopment a number of Medusse may be thrown 

 off from each Strobila. 



A considerable number of the Medusa pro- 

 geny having been separated, the Strobila stock 

 generally returns to its polypoid condition, 



Fig. 17. 



Production of Medusa; (Amelia aurita) from Po- 

 lype stock. (From Desor.) 



A, Medusa-form larvae on the stock above the 

 polype, which remains at the base, a. 



r,, lower surface of a detached Medusa. 



c, D, natural size. Young Medusae forming from 

 the polype above its disc. 



c 3 



