I20 PRIMATE ALVEOLINGUAL SALIVARY AREA 



b. The details of embryonic organization follow not only the bio- 

 genetic law of reveaUng rudiments of phylogenetically abandoned 

 stages, but they also offer evidence of progressive changes toward a 

 higher evolutionary type. Thus, in case of the early lesser subhngual 

 anlages, sprouting from a common epithelial keel, this attempted 

 mbrphogenetic advance of selected components of the entire group to 

 a higher and more specialized glandular t\-pe, is not a reversional 

 reminder of past phylogenetic conditions, but a temporary and evanes- 

 cent indication of a progressive tendency toward the acquisition of a 

 structurally more complex and, probably, functionally more efficient 

 organization. 



6. The inference lies at hand that the genetic cause of the inconstant 

 and variable occurrence of the greater sublingual (Barthohnian) 

 gland may be sought in tliis relation of the medial and lateral alveo- 

 Ungual areas. The former represents the earhest and oldest hne 

 along which specialization of the component elements led to the 

 establishment of the constant and largest major gland, the submaxil- 

 lary, with all its range of accessory and secondary glandular develop- 

 ment. The latter is still largely in the primitive condition, developing 

 numerous individual separate glands. Between the two is the 

 debatable intermediate field, occupied occasionally by the greater 

 sublingual gland and its duct. In the most thoroughly known primate, 

 man, this gland, when present, varies much in size and in the termina- 

 tion of its duct. The latter may open independently on the plica 

 subhnguaUs, or join the termination of the main submaxillary duct. 

 In many other human adults the gland does not develop. Individual 

 instances of its occurrence on one side and default on the other side of 

 the same subject are not uncommon. Its variable development in 

 the lower primates has already been described (Part III). In the 

 lower primate tv^pes, in which it is recorded as absent in the individuals 

 examined, and in the corresponding human variation, the lesser 

 subhngual group is usually large and extensive. The distinct impres- 

 sion is given, by the examination of a series of primate forms, that the 

 more medial elements of the lesser sublingual complex tend, in the 

 primate order, to become associated into a separate gland with a 

 common duct, the greater subhngual or Barthohnian component of 

 the entire alveohngual group. That this tendency has not yet 



