488 



EMBRYOLOGY. 



chondrification becomes observable in the membranous nasal capsule. 

 At that time the mucous layer of the epidermis, along a line that 

 extends from the median side of the eye directly to the nasal cavity, 

 undergoes proliferation and sinks into the underlying connective- 

 tissue layer as a solid ridge. Then from the nose to the eye the 

 ridge becomes constricted off, subsequently acquires a lumen, whereby 

 it is converted into a canal lined with epithelium, and opens out into 

 the nasal cavity. Toward the eye-end the canal is divided into two 

 tubules, which at the time of detachment from the epidermis remain 

 in connection with the conjunctival sac and suck up out of it tlr 

 lachrymal fluid. 



In Birds and Mammals, including Man (fig. 273), the place where 

 the lachrymal duct is located is early marked externally by a furrow 



which runs From the inner angle of 

 the eye to the nasal chamber. By 

 means of this furrow two ridges, which 

 play an important part in the for- 

 mation of the face, the maxillary 

 process and the outer nasal process, 

 are sharply marked off from each 

 other ; these will engage our atten- 

 tion later. According to COSTE and 

 KOLLIKER the lachrymal duct arises 

 by the simple approximation and con- 

 crescence of the edges of the lachrymal 

 groove. These older conclusions have 

 been contradicted by BORN and LEGAL, 

 one of whom has investigated Reptiles and Birds, the other Mammals. 

 According to them there arises, in nearly the same manner as in 

 Amphibia, through proliferation of the mucous epithelium, at the 

 bottom of the lachrymal groove an epithelial ridge, which becomes 

 detached but is not converted into a canal until a rather late period. 

 When we raise the question, how phylogenetically the lachrymal 

 duct may have first originated, we shall doubtless find that it has 

 been derived from a groove, by means of which the sac of the con- 

 junctiva and the nasal chamber are first put into connection. When, 

 therefore, we see the lachrymal duct established from the very begin- 

 ning simply as a solid ridge, as for example in the Amphibia, we 

 must call to mind how in other cases also originally groove-like 

 fundaments, such as the medullary furrow, make their appearance, 

 under special circumstances, as solid ridges. 



Fig. 273. Head of a human embryo, 

 from which the mandibular pro- 

 cesses have been removed to 

 allow a survey of the roof of 

 the primitive oral cavity. 



