42 THE DEVELOPMENT OF THE SALIVARY GLANDS IN MAN 



man gives rise to two constant structures, the orbital inclusion and 

 the parotid, and in addition suggest the possibihty of the occasional 

 presence of a third element, the orbitoparotid. The orbital inclusion 

 has a wide distribution in the mammalian series, having been found in 

 embryos of the cat, pig, mole, tree-shrew (Tupaia), chevrotain, and 

 opossum, in all of which it appears to be constant. The orbitoparotid 

 appears as a frequent variant in the cat, and indications of its presence 

 are found in Tupaia and Didelphys embryos, while the two ducts of 

 the parotid of Hyrax appear most easily explained on the assumption 

 that one of them is a highly developed orbitoparotid. If the "ductus 

 Stenonianus" of Hammar and Paulet is interpreted as referring to the 

 orbital inclusion, an outline of the history of this element may be con- 

 structed for the human embryo, agreeing in essentials with the more 

 detailed findings in the mammahan embryos above referred to. In 

 appearance it antedates the salivary anlages, being present in embryos 

 of 7.5 and 8 milhmeters, while the earliest undoubted parotid that has 

 been described is that of His's embryo Zw., which is already somewhat 

 advanced in development. The inclusion owes its origin to a process 

 of folding and constriction resulting in separation from the oral epithe- 

 lium, a process which may be conceived as advancing caudo-craniad if 

 Hammar's description of conditions in his 17 milHmeter embryo may 

 be taken as applying to the orbital inclusion. Here a duct with a 

 lumen lies parallel to the oral epithelium, to which, it may be inferred, 

 it is attached by its cranial extremity, in view of the fact that it was in- 

 terpreted as the parotid. If this be the explanation of the somewhat 

 perplexing findings in this embryo, the separation of the orbital in- 

 clusion in man is progressive caudo-craniad, as in the pig, and not 

 initiated at an intermediate point, as in the cat. The difficulty in thus 

 interpreting Hammar's account Ues in the fact that a younger embryo 

 (His's Zw.) has a well-marked parotid of a retort shape, affording ample 

 evidence of active prohferation in the anlage. To assign Hammar's 

 description to the orbital inclusion is to imply either that he has over- 

 looked the presence of a well-developed parotid anlage, or that his 

 embryo was abnormal in the suppression of that element. In this 

 connection it should be borne in mind that except in the older embryos 

 of Chievitz and in that of Bujard the orbital inclusion as a matter of 

 fact has been overlooked. The finding of the orbital inclusion in the 



