THE LATER DEVELOPMENT OF THE FROG 155 



About the time of hatching (6 mm.) a thick strand of cells 

 extends from each olfactory pit to the roof of the pharynx 

 just within the limit of the stomodseum (Fig. 52, B). Later 

 (9-12 mm.) each strand acquires a lumen which opens into the 

 olfactory sac, and also into the pharynx as the internal nares 

 or choance (Fig. 52, C). 



The extent of the olfactory cavity is increased by a cavity 

 formed in a dorsal cell proliferation of the olfactory epithelium. 

 This region soon forms a separate, dorso-lateral lobe which 

 disappears entirely during metamorphosis. 



The surface of the head pushes out above the olfactory sac so 

 that the duct leading to it is considerably elongated, and mean- 

 while, on account of the enlargement of the fore-brain and the 

 formation of the cartilage of the skull in this region, the 

 openings of the olfactory tubes or external nares are carried to 

 the dorsal side of the head, so that the entire olfactory organ 

 extends along a straight perpendicular axis (Fig. 52, D). 



During metamorphosis the olfactory organ becomes con- 

 siderably complicated by the appearance of various foldings 

 and out-pocketings of its walls, and by a sharp flexure of its 

 axis. We may mention only the more important of these 

 outgrowths. The first is an extension from the ventral side 

 of the olfactory chamber; here a solid mass of cells prolifer- 

 ates, acquires a cavity, and, enlarging rapidly, turns trans- 

 versely toward the medial side. This is the rudiment of 

 Jacobson's organ; a large glandular mass develops upon its 

 medial end. Other outgrowths are formed from parts of the 

 olfactory sac that are non-nervous, i.e., lined with indifferent 

 cells. One of these appears opposite Jacobson's organ and soon 

 becomes a large sac whose cavity is added to the olfactory 

 chamber. Another appears anteriorly at the base of the 

 olfactory duct; this receives the ducts of the lachrymal glands. 

 Later a dorsal sac grows out from the medial and posterior 

 walls of the tube. During late metamorphosis the axis of the 

 olfactory organ is sharply bent on account of the posterior 

 shifting of the internal nares. In addition to the glands of 

 Jacobson's organ, other glands appear as outgrowths of the 



