100 Trans. Acad. Sci. of St. Louis. 



lowed dorsally to receive the nasal sac; the narrow end 

 of the horn is in contact with the organ of Jacobson. 

 An antorbital process (Pr. ao.) is present on each side 

 and is chondrified throughout and connected by cartilage 

 with the trabecula. The line of union is easily distinguish- 

 able by the presence of young cartilage, and is just op- 

 posite the cephalic free margin of the trabecular crest. 

 The little rod of cartilage (Col. eth.) present in the 13mm. 

 larva dorsad of the olfactory nerve, has lengthened consid- 

 erably and reaches nearly to the trabecular crest. It extends 

 along the caudal two-thirds of the nasal sac between it and 

 the brain, inclining cephalo-ventrad to join by a broad 

 base the trabecula in front of the level of the anterior 

 trabecular plate. While the rod follows the dorso-mesal 

 edge of the nasal sac it, on the other hand, crosses the 

 surface of the olfactory bulb. Its ending is noteworthy: 

 it bends ventrally in a small but distinct hook (Pr. unc.) 

 just behind the olfactory bulb and a very short distance in 

 front of the corner of the Crista. Where the rod expands 

 to join the trabecula two processes arise ; one of these, the 

 Ethmoidal Process, (Pr. eth.) points dorso-mesad in front 

 of the olfactory bulb and approaches its fellow; the other 

 the Medial Nasal Process, (Pr. n. med.) grows cephalo- 

 ventrad along the internasal surface of the olfactory sac. 

 The latter process is separated from the end of the trabecula 

 and the horn by a notch, the Medial incisure (Inc. med.) 

 which allows the passage of the third main branch of the 

 ophthalmic V (Fig. 4, Oph. V,). Another little spur of 

 cartilage on the rod in question, the Lateral Process (Pr. 

 lat.), is present near the hinder end and reaches out into the 

 perirhinal membrane of the dorsal side of the nasal sac. 

 No separate centers of chondrification in this membrane were 

 discovered either in this or in subsequent stages. 



Reviewing the cartilaginous rod (Col. eth.) at this stage, 

 it is seen to arise from the trabecula just in front of the 

 anterior trabecular plate by a thick base from which two pro- 

 cesses spring, then to arch backward across the olfactory bulb 

 and over the olfactory nerve to end free in a ventrally turned 



