TEANSVERSE SECTIOKS 23 



which the thread is only partially protruded, and 

 in which the relations of the capsule to the thread 

 can be readily determined. 



2. The mesoderm may be seen in teased preparations ia the 



form of transparent shreds or sheets of greater or 

 less extent, in which a distiact longitudinal striation 

 is present, caused by the muscular tails of the large 

 ectoderm cells. 



3. The endoderm cells are very variable in shape. Each 



commonly contains one or more vacuoles which may 

 be so large as to reduce the wall of the cell to a very 

 thin lamella. The outer part of the cell, next the 

 mesoderm, contains in Hydra viridis a number of 

 small spherules coated with chlorophyll, to which 

 the green colour of the animal while living is due. 

 In the brown species of Hydra similar bodies are 

 present, but devoid of chlorophyll. 



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 prepoA-e these kill the animal in the expanded 

 condition by suddenly irrigating it with osmic acid; wash 

 it well in distilled water j transfer to alcohol; stain with 

 picrocarmine, and then imbed in paraffin and cut with 

 a microtome. 



1. The ectoderm. 



a. The large ectoderm cells are well seen in such 

 sections ; their muscular tails, which are maioly 

 longitudinal ia direction, are cut transversely, 

 and appear as a row of highly refractive dots 

 on the outer surface of the mesoderm lamella. 

 At the basal end or foot of the Hydra the 



