104 Trans. Acad. Sci. of St. Louis. 



long, branched, tubular Jacobson's gland with wide lumen 

 and rather thick walls. The end of this gland occupies a 

 position close to the ventral cephalic corner of the olfac- 

 tory window, and in the rest of its extent is situated between 

 the main-sac and the upper surface of the Cornu trabeculae 

 in the space (Sub. s) already mentioned. 



The differences that are noticed in the cartilaginous frame- 

 work over the preceding stage are due chiefly to growth 

 of existing parts ; there are no new centers of chondrification. 



The cranial floor in the olfactory region is broader, the part 

 made by the anterior trabecular plate giving support to the 

 bulbs. The olfactory window is of the same size as it was in 

 the third stage ; the opening is therefore relatively smaller 

 and begins to assume the appearance of the adult olfactory 

 foramen. The part of the capsular wall occupied by the 

 olfactory foramen is obliquely placed and faces forward and 

 outward. Since the nasal sac has increased in length very 

 much, extending in half its length beyond the cranial cavity 

 and ending behind the olfactory foramen, it happens now, 

 for the first time, that a part of the cartilaginous cranial side 

 wall, made by the Crista trabeculae, stands between the brain 

 and the sac. 



There is a complete cartilaginous wall separating the 

 cranial cavity and internasal space, stretching from side to 

 side at the level of the anterior margins of the olfactory for- 

 amina. Although it is thin in its middle part, this partition, 

 which I have called the Ethmoidal Plate (Eth. pi.) in the 

 same sense that Gaupp* uses the term, is chondrified 

 throughout. Its caudal surface slopes gently into the dorsal 

 surface of the anterior trabecular plate without interruption. 

 The cephalic surface of the ethmoidal plate is thickened along 

 its dorsal and lateral margins corresponding to the positions 

 of the ethmoidal bridge and base of the rod (Col. eth.) in the 

 earlier stages of development. At these places the growth of 

 cartilage in the mesenchymal tissue of the internasal space 

 appears to be most active. In later stages (30-33mm.) as a 



* Morph. Arbeit. 2, p. 313. 



