THE SENSE OEG-ANS. 



185 



1. The Nose. 



The olfactory organs appear at a very early period of develop- 



FlG. 66.— Diagrammatic horizontal section of a 12 mm. Tadpole, at the time of 

 appearance of the hind limbs. The plane of section of the right side of 

 the head is taken at a more dorsal level than that of the left side, x 30. 



A, aorta. AC, carotid artery. ACE, pharyngeal artery. ACI, anterior cerebral 

 artery. AP, pulmonary artery. AT cutaneous artery. EF.l, EP.2, EP.3, EP.4, 

 efferent brancliial vessels of first, second, third, and fourth branchial arches. GM, 

 glomerulus. HC.l, first branchial cleft. KA, right segmental duct. KA', hinder 

 end of left segmental duct. KP, head kidney. KS, nephrostome. LC, laryngeal 

 chamber. LG-, lung, OC. optic cup. OF, olfactory pit. OL, lens. PA, pancreas. 

 PD, pancreatic duct. TD, duodenum. TI, intestine. 



ment, about the time of closure of the brain, as a pair of thicken- 

 ings of the nervous layer of the epiblast at the anterior end of 



