22 HYDRA 



In each ovary only a single ovum is produced. 

 The ovary is formed like the testis by multiplica- 

 tion 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 nutriment. The 

 ovum is at first amoeboid so as to come into 

 contact with as many of the nutrient cells as 

 possible. When it has reached its full size it 

 becomes spherical or ovoid ; the capsule then 

 thins aways 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 into a young Hydra. 



II. EXAMINATION OF PEEPAEED SPECIMENS. 

 A. Teased Specimens. 



Tease thoroughly in a mixture of equal parts of glycerine 

 and water a small portion of the body-wall of a Hydra that 

 has been treated for a day either with Mutter's fluid or with 

 a mixture of osmic and acetic acids. Cover, and examine 

 with a high power. 



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 



