PREPAE.ED SPECIMENS. 23 



proclueed. 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 other cells of the ovary, which 

 form a capsule around it and supply it with nutri- 

 ment. 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 OP PREPARED SPECIMENS. 



A. Disintegrated Specimens. 



Kill a Hydra luith 1 per cent, acetic acid; then add a 

 large drop of water, cover, and examine with loiu and high 

 powers. 



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 lie on the outer surface of the supporting 



