IV DEVELOP>rENT 303 



are formed in it, and in Hydra viridis it also acquires 

 green chromatophores. 



When the ovary is ripe the ovum draws in its pseudopods 

 and takes on a spherical form : the investing layer then 

 bursts so as to lay bare the ovum and allow of the free access 

 to it of the sperms. One of the latter conjugates with the 

 ovum producing an oosperm (p. 198) or unicellular embryo. 



The oospeim undergoes segmentation, dividing into 

 a number of cells which constitute a morula or polyplast 

 (p. 200), the outermost cells of which become changed into 

 a hard shell or capsule. The embryo, thus protected, falls 

 to the bottom of the water, and after a period of rest 

 develops into a Hydra, its cells becoming differentiated 

 into ectoderm and endoderm, the enteron and mouth 

 being formed, and the tentacles budding out round the 

 latter. 



It was stated on p. 301 that in a budding Hydra the buds 

 do not always become detached at once, but may them- 

 selves bud while still in connection with the parent, 

 temporary colonies being thus produced. 



Suppose the state of things to continue indefinitely : the 

 result would be a tree-like colony or compound organism 

 consisting of a stem with numerous branchlets each ending 

 in a Hydra-like zooid. Such a colony would bear much the 

 same relation to Hydra as Carchesium or Epistylis bears to 

 A^orticella (p. 281). 



As a matter of fact this is precisely what happens in a 

 great number of animals allied to Hydra and known by the 

 name of Zoophytes or Hydroid polypes. 



Every one is familiar with the common Sertularians of the 

 sea-coast, often mistaken for sea-weeds : they are delicate, 

 much-branched, semi-transparent structures of a horny 



