THE SYRINX OF GALLUS DOMESTICUS 191 



sterno-tracheal muscles, reach the trachea small additional masses 

 of developing muscular tissue extend caudad along the dorso- 

 lateral and ventro-lateral aspects of the trachea. These small 

 additional masses seem to arise from, and certainly do not differ 

 from, the tissue found in the developing sterno-tracheal muscles. 



In the succeeding stages the muscles continue to develop by 

 the addition of more cytoplasmic bundles. The nuclei of the 

 muscle cells are easily distinguished from those of the developing 

 connective tissue cells, which are present between the muscular 

 bundles, for the former are more elongated and regular in shape. 

 No other structural changes call for special mention until the 

 296th hour is reached, when faint cross striations appear in cer- 

 tain fibers. These, however, are quite rare. 



At 452 hours the fibers have arranged themselves into more 

 definite bundles. The fibers of these bundles show distinct cross 

 striations, and numerous elongated nuclei appear to lie on or 

 between them. Certain of these fibers are attached directly to 

 the perichondrium of the lower tracheal rings. 



5. Inter cartilaginous membranes 



When the aggregated mesenchymal cells arrange them.selves 

 to form the anlagen of the skeletal elements, small spaces exist 

 between these anlagen. These spaces, also, are occupied by typi- 

 cal mesenchymal cells, but here they are fewer and farther apart 

 (fig. 13). This condition is particularly well marked in a 248 

 hour embryo, in which it occurs between all of the developing 

 cartilages except the first three intermediate syringeal cartilages. 

 The widest spaces are those between the first half-rings and the 

 fourth intermediate sjTingeal cartilages. In these spaces the 

 mesenchyma shows a slight condensation, suggesting the arja- 

 gen of the external tympanic membranes. In a 260 hour embryo 

 the cells lying near the periphery of this developing membrane 

 have begun to elongate. Differentiation becomes more marked 

 in each of the succeeding stages (272, 284, 296, 308, and 320 

 hours), but it is not until the 332 hour stage that small connective 

 tissue fibers can be disclosed by the use of Mallory's stain. 



