DEVELOPMENT OF SALIVARY GLANDS IN THE DOMESTIC CAT 255 



While the material and the means of forming a continuous fold are 

 thus present, we are naturally led to seek the occasion of its interrup- 

 tion. This, I believe, is to be found in the space requirements of the 

 masseter muscle. Its anlage is a condensation in the mesenchyme 

 shield-shaped in section (Figs. 36-38, jj) and at first continuous with 

 that of the zygoma. In embryos of 13.5 milUmeters the myoblasts can 

 be distinguished and begin to be separated by a line of less dense tissue 

 from the fibroblasts of the latter. The ental surface of the masseter 

 is nearly sagittal in direction, at least from the orbital angle craniad. 

 The dorsal border is approximately horizontal. To it the cranio- 

 ventral or sloping border, which is broad and rounded, ascends ob- 

 liquely, with the result that the muscle has a tapering extension along 

 the zygoma. This border crosses the buccal sulcus in the orbito- 

 parotid interval. Here, also, the muscle comes closest to the sulcus, 

 which, from the orbital angle to the postangular bend, has a straight 

 course, nearly sagittal, but with a slight lateral deviation, while from 

 the postangular bend to the angulus oris the sulcus is obliquely trans- 

 verse (Figs. 35, 97). The sulcus is then passing craniad of the border 

 of the muscle with a lateral sweep, at the point where the latter ascends 

 dorsal to it. The tapering prolongation of the masseter along the 

 zygoma lies dorsal to the maxillomandibular plane in the region of the 

 parotid (Figs. 51-55). The approximation of sulcus and muscle is 

 here also appreciable in the region of the interval (Figs. 56-57). It 

 must be borne in mind that at the stage we are considering, 1 2-13.5 

 millimeters, the sagittal segment of the sulcus is still short, the folds 

 separated only by a minute interval (Figs. 31, 34), which markedly 

 increases in succeeding stages (Figs. 41, 43-45). That the muscle is 

 close enough to make its presence felt upon the sulcus through the 

 intervening mesenchyme, is shown by the ventral deflection of the 

 parotid flange, of the orbitoparotid bridge when present, and of the 

 cranial prolongation of the orbital inclusion itself. 



The anlage of the masseter, as of the other muscles of mastication, 

 defines itself close to the trunk of the mandibular nerve and thence 

 extends craniad, overlapping more and more of the buccal sulcus as 

 it does so. The direction of growth being sagittal, the muscle is 

 brought into more intimate relations with the laterally diverging sul- 

 cus as it advances, and a juxtaposition of its sloping border with the 



