DEVELOPMENT OF THE SENSE ORGANS 



373 



are developed for its support (Fig. 352). The organ of Jacobson is not func- 

 tional in man but in many animals constitutes a special olfactory organ. 



The Conchas are structures which are poorly developed in man. They appear 

 on the lateral and median walls of the primitive nasal fossae. The inferior concha, 

 or maxillo-turbinal, is developed first in human embryos (Figs. 352 and 353). It 

 forms a ridge along the caudal two-thirds of the lateral wall and is marked off by a 

 ventral groove which becomes the inferior nasal meatus. The naso-turbinal is 

 very rudimentary and appears after the fourth month as a slight elevation dorsal 

 and cranial to the inferior concha (Fig. 354). Dorsal to the inferior concha 

 arise five ethmo-turbinals, which grow smaller and are located more caudad as we 

 pass from the first to the fifth (Fig. 354). According to Peter, the ethmo-tur- 



Horizonfal ethmoid plate 



Lateral lame/la 



Sphenoid 



Hard palate 



Upper lip 



Fig. 354. — Right nasal passage of a fetus at term (Killian) 



ethmo-turbinals. 



•Soft palate 



I, maxillo-turbinal: II-IV, 



binals arise on the median wall of the nasal fossa and, by a process of unequal 

 growth, are transferred to the lateral wall (Fig. 353). Accessory conchse are 

 also developed, according to Killian. 



In addition to the ridges formed by the conchas, there are developed in the grooves between 

 the ethmo-turbinals the ethmoidal cells. The frontal recess gives rise to the frontal sinus. At 

 the middle of the third month the maxillary sinus grows out from the inferior recess of the first 

 groove. The most caudal end of the nasal fossa becomes the sphenoidal sinus, which, as it 

 increases in size, invades the sphenoid bone. 



The cells of the olfactory epithelium become ciliated but only a small area, representing 

 the primitive epithelial invagination, functions as an olfactory sense organ. The olfactory cells 

 of this area give rise to the olfactory fibers which constitute the nerve. The development of 

 this has been described on page 355. 



IV. The Development of the Eye 

 The anlage of the human eye appears in embryos of 2.5 mm. as a thickening 

 and evagination of the neural plate of the fore-brain. At this stage the neural 



