THE H YD ROM ED USAE 



stage they appear in the bud of the gonophore at an early phase 

 of its development ; in the female Podocoryne, for instance, they 

 have been first noticed in the endoderm of the bud, and migrate 

 into the ectoderm of the manubrium through the mesogloea ; in 

 Coryne, where no entocodon is formed, they both appear and ripen 

 in the endoderm of the sporosac. In the third stage they first 

 appear in the tissues of the hydroid, blastostyle, coenosarc, or 

 hydrorhiza, from which the gonophore will ultimately be budded ; 

 in Hydractinia, for example, they are first noticeable in the ectoderm 

 of the blastostyle (Collcutt, 26), and migrate along the endoderm 

 into the sporosac, breaking through the mesogloea to ripen in the 

 ectoderm. In the female Eudeiidrium racemosum their wanderings 

 are still more complex. They are formed in the ectoderm of a main 

 hydroid, migrate into the ectoderm of a lateral h3'droid, thence 

 into the endoderm, first of the blastostyle, then of the sporosac, 

 and ultimately break through into the ectoderm of the sporosac. 

 Although in many cases the generative cells are only recognisable 

 for the first time in the endoderm, it is probable that they are 

 in all cases originally ectodermal cells, which may or may not 

 migrate into the endoderm ; in almost every instance they ripen in 

 the ectoderm. The whole question is dealt with by AVeismann 

 (10). 



There is thus evidence that a marked change is in progress 

 among the Anthomedusae ; the alternation of the fixed nutritive 

 hydroid with the sexual free-swimming medusoid is being gradually 

 abandoned ; the medusoid, the function of which was to form, 

 ripen, and disseminate the generative cells, is being replaced by the 

 sporosac, in which they merely ripen ; their formation is becoming 

 a function of the colony or of the hydroid. Curiously enough, in 

 one genus, Dicoryne, which forms sporosacs of the simplest type 

 on a blastostyle, there occurs an apparent reversion to the old 

 method of dispersal of the species, for the sjDorosac becomes con- 

 stricted oft" from the blastostyle, and swims freely by means of 

 strong cilia (Allman, 1). 



Asexual Keproduction. — In geynmation, which is the rule 

 among hydroids, both ectoderm and endoderm form a hollow lateral 

 protrusion of the body or coenosarc ; this absorbs a window in the 

 perisarc where necessary, and either by the development of mouth 

 and tentacles becomes a new hydroid, or in the manner already 

 sketched (pp. 18, 19) is converted into a medusoid or a sporosac. 



Gemmation from a medusoid appears to be of a similar 

 " laminar " character, and to follow the lines sketched on pji. 18, 19 ; 

 its product is always a medusoid. 



Fission is rare among hydroids ; it may be transverse (Pro- 

 tohydra) or longitudinal (Polypodium). It has not been shown to 

 occur among medusoids in this group. 



