314 JAMES EOLLTN SLONAKER 



By the ninth day of incubation the extrinsic muscles have 

 become much more sharply defined, both as to shape and con- 

 nections (figs. 69 to 72, M). These cells, though they show 

 great elongation and definite parallel arrangement, still lack 

 the differentiation of striated muscle. At this age the wall of 

 the orbit is cartilaginous, very closely resembling the cartilage 

 cells of the sclerotic coat. 



Although these formative muscle masses increase in extent 

 to keep pace with the rapidly increasing size of the eyeball, 

 there is no appearance of striation in the muscle until two days 

 after hatching. At this age very faint and indistinct cross 

 striations are barely visible, but by the fourth day after hatching 

 they are more easily demonstrated, but are still far from complete. 

 The muscles of the upper and lower lids, the suprapalpebral 

 and infrapalpebral, show occasional faint striations, most pro- 

 nounced in the infrapalbebral near its proximal attachment to 

 the fid. 



About the sixth day after hatching all the extrinsic muscles 

 are clearly striated. The significance of this is easily under- 

 stood when we consider the fact that the eyes of the nestling 

 open about this stage. Previous to this time there was no occa- 

 sion for physiological activity of these muscles. It is also inter- 

 esting to note that the infrapalpebral muscle, which is the principal 

 one involved in the act of opening the eye of the bird, becomes 

 striated earlier than the suprapalpebral muscle. 



hitrinsic muscles. The intrinsic muscles of the eye — the 

 cihary muscles and the muscles of the iris — appear at a much 

 later stage than the extrinsic. Although the formation of the 

 iris has fairly well advanced by the twelfth day of incubation 

 and the ciliary bodies are beginning to show, no indication of 

 muscle tissue can be observed. The cells of these structures 

 still lack differentiation. 



At the age of hatching a few spindle-hke cells appear between 

 the ciliary bodies and the sclerotic coat. They are fairly defi- 

 nitely arranged and seem to have been derived from the same 

 cells as those which form the sclerotic coat. These later develop 

 into the ciliary muscles. 



