308 HYDROID POLYPES CHAP. 



fragments into which they become broken up, thus 

 continually increasing in size at their expense. Ulti- 

 mately 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 

 themselves 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 



