22 HYDRA 



contact with as many of the nutrient cells as pos- 

 sible. When it has reached its full size it becomes 

 spherical or ovoid ; the capsule then thins away 

 and ruptures at its most prominent part, 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 which lie 

 in the mesoderm lamella, and on which the con- 

 tractility of the body depends. 



b. The interstitial cells are much smaller, and usually 



remain in small clusters. Within some of them 

 the early stages of development of the nematocysts 

 may be seen. 



c. The nematocysts. In specimens killed with Miiller's 



fluid, nematocysts are frequently met with in 



