268 DEVELOPMENT OF SALIVARY GLANDS IN THE DOMESTIC CAT 



craniad as far as the end of the superior dental anlage. The advance 

 of the duct is thus early initiated. In its caudal sections the anlage 

 appears constricted at its attachment, but as yet none of it is free from 

 the oral epithelium. 



In the 25 miUimeter embryo the first orbital forms a keel 133 /x in 

 length, but with its extremities still in the same relative position, 

 craniad at the end of the dental ridge, caudad at the level of the deep 

 facial vein where it crosses the sulcus. It is composed of deeply 

 staining cells resembling those of the basal layer of the oral epithelium, 

 forming a small solid keel constricted caudad along its line of attach- 

 ment, but not yet free in any part of its extent. Its growth, therefore, 

 has only sufi&ced to keep pace with the general lengthening of the oral 

 cavity, and no independent progress has been made as yet. In this 

 embryo the tip of the orbital inclusion slightly overlaps the anlage at 

 its caudal extremity. 



In the 28 millimeter embryo some advance has been made. Three 

 orbitals are present on the right side, two on the left. On both sides 

 the second hes opposite the deep facial vein. On the right side the 

 first orbital has a total length of 320 fi, of which all but one section 

 (20 /x) belong to the attachment. This begins as a low keel near, but 

 not at, the fundus of the buccal sulcus, as in the earlier embryos. It 

 gradually increases in height caudad, and in its last third is constricted 

 at its base. In a few sections near its cranial extremity a minute fissure 

 indents the base of the flange. It has been seen that the fissure was 

 not present in the earliest stages of the anlage, which, therefore, does not 

 owe its inception to a flange. Two interpretations of such a fissure 

 are possible and perhaps not mutually exclusive. First , it may be due to 

 the movements of the cells incident to proliferation and the formation 

 of the keel ; second, it may be a rudimentary fragment of the superior 

 alveobuccal sulcus which in this region becomes continuous with the 

 secondary buccal. This is the region of transition between the seg- 

 ment of the mouth which retains the maxillomandibular plane as a 

 permanent element of the oral cavity, and that which suppresses a 

 portion of it as the orbital inclusion. The transition is craniad of the 

 orbital angle, corresponding to the cranial process of the inclusion. 

 Here, then, small irregularities of the developmental process might be 

 expected to occur. 



