The structural characteristic aspects of the masticatory muscle spindles in the pangolin were investigated electron microscopically. The results obtained were as follows:
1 . Each muscle spindle consists of a thin outer capsule and intrafusal muscle fibers ranging from one to seven in number and occupied by abundant collagenous fibrils.
2. The outer capsule consists usually of two lamellated capsule cells and sometimes of more than two, that is, four at the neuro-vascular passage.
3. The periaxial lymphatic space is very wide, being occupied by many blood capillaries, myelinated nerve fibers and abundant collagenous fibrils.
4. The inner capsule consists of one lamellated cell, being devoid of the basement membrane and continuous with the innermost layer of the outer capsule. This cell envelopes each of the intrafusal muscle fibers with its branched cytoplasmic processes, by which the intraaxial space is subdivided into compartments, one for each muscle.
5. The intrafusal muscle fibers showed an undulated contour. Beneath the sarcolemma were found specific laminated structures, elaborate membranous networks of the T. system and elaborate sarcoplasmic processes having glycogen-like granules and projecting into the caverns packed with homogeneous filaments. These structures may be of reductive products following the disuse atrophy of the masticatory muscle resulting from the poor jaw movement.
6. Many satellite cells were seen, some of which appeared to anchor two adjacent intrafusal muscle fibers.
7. The sensory terminals were classified into the annulospiral (primary) type and the flower-spray (secondary) one, the former being smaller in number and size than usual and the latter being dominant. The sensory cross-terminals are found to be frequent.
8. The gamma motor end-plate was seen near the polar region of the intrafusal muscle fibers.
9. Blood capillaries enter the periaxial lymphatic space through the spindle capsule to reach the axial space. Some of them enter the periaxial space through the polar aperture of the capsule.
In summary of the results mentioned above, it can be emphasized that the masticatory muscle spindles of the pangolin showed a tendency of atrophy. In other words, they still retain the traces of their youthful features (their early stage of development) . This phenomenon would be highly relative to the undeveloped masticatory mechanism in the pangolin.