76 ADRENALIN IN ANNELIDS 



them. Wherever they were found these chromaffin cells were always 

 constant in number and similar in position in the ganglion, six being 

 always present arranged in three groups of two each, a ventral and 

 two lateral groups. The two cells of the ventral group were usually 

 the largest. The size of the cells varied greatly in the different 

 animals, being small in the two polychastes and reaching the largest 

 size in Hirudo medicinalis, which was therefore selected for further 

 investigation. 



In this animal the two ventral cells are of very large size; they have 

 been called colossal or giant cells by Retzius^^ and others. The lateral 

 group on each side consists of two smaller cells which lie respectively 

 just posterior to the anterior and posterior lateral nerves; their posi- 

 tions are indicated in Fig. 1. These six cells have all the appearance 

 of nerve cells, possessing processes which run out in the lateral nerves 

 and staining similarly to the other nerve cells of the ganglion. It was 

 found that the six cells could be very clearly demonstrated by staining 

 the freshly excised ganglion with methylene blue, and then irrigating 

 the preparation mounted in water under a cover-slip with a dilute 

 bichromate solution; all the nerve cells became bleached by this 

 process with the exception of the six chromaffin cells which retained 

 the blue stain. The probable explanation of this reaction is that the 

 bleaching action of the chrome salt is prevented in the chromaffin 

 cells by its combination with the chrome-staining substance, thus pro- 

 tecting the methylene blue stain. The nerve cells of the ganglion of 

 the leech are divided into groups, shut off from one another by septa, 

 whose arrangement is shown in Fig. 1. The individual nerve cells are 

 unipolar and hang free in their particular compartment lying in a 

 clear nutritive fluid; their relative positions are therefore liable to 

 alterations in the compartment, for instance the two giant cells can 

 be moved about freely by pressure on the cover-slip in a suitable prep- 

 aration. The nerve cord itself is suspended in a blood space known 

 as the ventral sinus, the thin sheath of the ganghon intervening only 

 between the compartments in which the nerve cells lie and the sur- 

 rounding blood. It is quite possible that an interchange takes place 

 between the fluid in which the nerve cells are suspended and the blood 



"Retzius, G., Biol. Unlersuch., 1891, ii, 13. 



