PREPARED SPECIMENS. 27 



is produced. The ovary is formed, like the testis, 

 by multiplication of the interstitial cells to form 

 a small swelling ; one of these cells, near the 

 centre of the heap, soon becomes larger than the 

 rest and becomes the ovum. This grows rapidly 

 at the expense of the interstitial cells of the 

 ovary, which form a capsule around it and supply 

 it with nutriment. The ovum is at first amoeboid, 

 so as to come into contact with a large number 

 of the nutrient cells. When it has reached its 

 full size it becomes spherical or ovoid ; the 

 capsule then thins away and ruptures at its 

 summit, and, shrinking back, leaves the outer 

 half of the ovum exposed to the water, and 

 ready to receive the spermatozoa. 



After fertilisation the ovum segments, forms 

 a hard capsule around itself, becomes detached 

 from the parent, and, falling to the bottom of 

 the tank or pond in which the animal is living, 

 develops after a time into a young Hydra. 



II. EXAMINATION OF PREPARED SPECIMENS. 



A. Disintegrated Specimens. 



Kill a Hydra with 1 per cent, acetic acid and a trace of 

 osmic acid. Leave it for five minutes, and then examine in 

 water. 



The cells are loosened from one another, and may now be 

 examined separately. 



1. The ectoderm cells. 



a. The large ectoderm cells will be found isolated in 

 various parts of the preparation. They are some- 

 what conical in shape, their outer ends being 

 broad and containing the large nuclei. Their 

 inner ends are much narrower, and are produced 

 into slender muscular processes or tails. These 

 tails He on the outer surface of the supporting 



