308 HYDROID POLYPES CHAP. 



Ultimately the ovary comes to consist of this single amoeboid 

 ovum and of a layer of superficial cells forming a capsule for 

 it. As the ovum grows, yolk-granules (p. 195) are formed 

 in it, and in Hydra viridis it also acquires Zoochlorellae 



(P- 304)- 



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 oosperm 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, which eventually bursts and sets 

 free the embryo. The embryo develops into a Hydra, its 

 cells becoming differentiated into ectoderm and endoderm, 

 the enteron and mouth being formed, and the tentacles 

 budding out around the latter. 



It was stated on p. 306 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 

 Vorticella. 



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 Hydroids known 



