I.—PHYSIOLOGY. 207 
muscles of the insect. As Caullery points out, however, the supposed 
spores closely resemble Microsporidia, and Portier’s interpretation may 
be erroneous. In this category also belong the symbionts described as 
occurring in Glossina by Roubaud (1919) and before him by Stuhlmann, 
these being found in certain hypertrophied cells of the intestinal epithe- 
lium. When liberated into the gut lumen, the symbionts are stated 
to multiply by budding after the manner of yeasts. Roubaud regards 
the yeasts as fungi, allied to the Cicadomyces of Sule, and finds that 
they are transmitted hereditarily from the adult to the egg, larva and 
pupa. 
Group IV.—Intracellular symbionts of deep tissues. ‘This group 
of symbionts is most frequently found in insects, but their nature was 
not disclosed until recent years. Already, in 1858, Huxley described 
an organ which is constantly present close to the ovary in Aphis. 
Balbiani (1866) named it the pseudovitellus, or green body, and 
Metchnikoff (1866), who followed its development, named it ‘ secondary 
vitellus.’ The function and structure of this organ were studied by 
subsequent authors without being understood until, in 1910, there 
_ appeared two important papers by Pierantoni (February 6), and Sule 
(February 11), who demonstrated their symbiotic character, recognising 
the intracellular inclusions as yeasts whose evolution they completely 
followed. ‘Their results have been confirmed by various authors, 
especially by Buchner, who in a remarkable series of papers describes 
a number of associations existing between insects and micro-organisms 
and reaches important generalisations as to their significance. It is 
from a collective work on the subject by Buchner (1921) that most of 
our information regarding this class of symbionts is taken. 
Among the symbionts of deep tissues in insects are found a whole 
series of specialisations among the host-elements harbouring the 
symbionts. The least specialised instance is represented by Lecaniine 
where the yeasts are distributed throughout the body (perivisceral fluid, 
cells of fat-body); the fat-body cells may be regarded here as facultative 
Mycetocytes. In cases like Orthezia, symbiotic bacteria occur in certain fat 
cells which still contain fat droplets ;this condition is also found in certain 
Cicadas, the yeasts being contained in fat cells which continue to 
accumulate fat, glycogen and urates. Finally cases occur as in Blattids 
where symbiotic bacteria are found in special cells greatly resembling 
fat cells but already forming well differentiated Mycetocytes. This 
_ class is well represented in and about the digestive tract of Pediculidee 
~ (Hematopinus) and certain ants (Camponotus). Still more advanced 
in specialisation are those cases in which the symbiont-containing cells 
 (Mycetocytes) agglomerate to form true organs termed Mycetomas, 
organs that are surrounded by flattened epithelial cells, the component 
mycetocytes containing either yeasts or bacteria as symbionts; such 
eases are found in Aphids, Chermids and Aleurodids. Mycetomas may 
occur singly or in numbers according to the nature of the host; the 
epithelial covering of the organ varies in its cell structure and pig- 
mentation, and the organ may be plentifully supplied with trachee 
whose finest branches penetrate into the interior of the mycetocytes. 
The relations hetween the mycetocytes or mycetomas and the other 
1923 Q 
