TRANSVERSE SECTIONS. 29 



The endoderm cells may also contain, in addition 

 to particles of food, little clusters of brown or black 

 granules, probably formed by breaking down of the 

 chlorophyll grains. The flagella are seldom pre- 

 served in teased specimens. 



B. Transverse Sections. 



Examine a series of transverse sections through the body 

 of Hydra. To prepare these, kill the animal in the expanded 

 condition by Perenyi's fluid ; transfer first to 70 per cent, and 

 then to 90 per cent, alcohol; stain with hcematoxylin, and 

 then imbed in paraffin and cut it into thin sections with a 

 microtome. 



1. The ectoderm. 



a. The large ' ectoderm cells are well seen in such 



sections ; their muscular tails, which are mainly 

 longitudinal in direction, are cut transversely, 

 and appear as a row of highly refractive dots on 

 the outer surface of the mesoglcea. 



At the basal end or foot of the Hydra the 

 ectoderm cells are more columnar in shape, 

 contain longitudinal rows of granules, and are 

 probably glandular. 



On the outer surface of the ectoderm cells 

 there is a thin cuticular layer, more obvious in 

 the brown than in the green Hydra. 



b. The interstitial cells are well seen in sections through 



the body or tentacles, but are absent in the foot. 



c. The nematocysts are very abundant in the tentacles, 



less numerous in the body, and absent in the 

 foot. In specimens killed with osmic acid, few 

 if any of them will be discharged. 



d. The testis or ovary may be cut across, and should 



be looked for. 



E 



