290 



COMPARATIVE ANATOMY 



correspond to true sebaceous glands developed in connection with 

 hair-rudiments, the hairs disappearing but the glands remaining. 

 They are wanting in some Mammals (e.g. Monotremes, Armadillo, 

 Manis, Dolphin, Seal, Elephant, Camel). Certain modified sweat- 

 glands known as the glands of Moll are also present within the 

 eyelids of Mammals, opening on the margins of the lids close to 

 the eyelashes. 1 



AUDITORY ORGAN. 



Certain relations to the integumentary sense-organs of Fishes 

 and Amphibians can be traced in the organs of taste (p. 254), and the 

 same is true of the auditory organ, the function of which is con- 

 cerned with equilibration as well as with 

 hearing : in all three cases the sensory 

 epithelium is derived directly from the 

 ectoderm. 



The first trace of the auditory organ 

 is seen as a thickening of the ectoderm 

 (auditory plate) on either side of the 

 primary hind-brain between the trigeminal 

 nerve and vagus group, which becomes 

 invaginated and separated off to form a 

 vesicle (Fig. 211), the epithelium of which 

 eventually undergoes differentiation into 

 elongated cells of sensory epithelium pro- 

 vided with hair-like processes, and sup- 

 porting cells (Fig. 212), as in that of 

 the integumentary sense-organs referred 

 to above. The sensory cells are in rela- 

 tion with nerve-fibres which arise from 

 the auditory ganglion. 



After the vesicle of either side has 

 become separated off from the ectoderm, 

 it sinks deeper and deeper into the meso- 

 dermic tissue of the skull, loses its original 

 PFiform or rounded shape, and becomes 

 divided into a superior and an inferior 

 p ar t ? called respectively the utriculus and 

 sacculus, at first connected with one another 



by a wide utriculo-saccular canal (Fig. 213). From the former the 

 so-called semicircular canals become developed, while from the latter 

 the tube-like ductus endolymphaticus and the lagena (cochlea) are 

 formed. 



1 In Cetaceans the whole lacrymal apparatus has undergone degeneration in 

 adaptation to the external conditions, and the nictitating membrane is vestigial : 

 well-developed lacrymal and Harderian glands are present in the embryo. A 

 greater or less reduction of the lacrymal gland may also occur in other Mammals, 

 e.g. Seal, Hippopotamus, Elephant, Otter, and Mole. 



FIG. 211. HEAD AND AN- 

 TERIOR PORTION OF BODY 

 OF A CHICK. (In part 

 after Moldenhauer. ) 



A, eye; LB, primitive 

 auditory vesicle seen 

 through the wall of the 

 head ; RG, olfactory pit ; 

 f, point at which the ex- 

 ternal auditory passage 



to IV, first to fourth 

 visceral arches. 



