622 Jacob Parsons Schaeffer. 



developed sufficiently so that they may be termed the primitive 

 nasal fossae. The fossae communicate freely with the exterior 

 by means of the nares, but, in the absence of choanse, they end 

 blindly at their dorsal and inferior terminations (fig. 2). They 

 are rather widely separated at this time (fig. 1). A reference to 

 fig. 2 will show both the lateral and medial walls of the left nasal 

 fossa of a 35-day embryo. It will be noticed that the lateral wall 

 is extremely simple, presenting merely a plane surface. The medial 

 or septal wall is slightly grooved and the groove overhung by a 

 fold. This is the anlage of the organon vomeronasale (Jacobsoni). 

 Note also that the fossa ends blindly and that there is no connec- 

 tion between the oral and the nasal fossa at this time. The point 

 of the primitive choana (posterior naris) is nearly thinned out 

 sufficiently to represent the membrana bucconasalis (fig. 5). 



The primitive choance 



The dorsal extension of the blind pouches or primitive nasal 

 fossae continues until the ectoderm of the nasal fossae meets the 

 ectoderm of the oral fossa. We have now in these positions merely 

 thin membranes composed of two layers of abutting epithelium — 

 nasal and oral — separating the dorsal portions of the primitive 

 nasal fossae from the oral cavity. These membranes, which have 

 been carefully studied and named by Hochstetter, the "Mem- 

 branaebucconasales," (fig. 5) become so attenuated and thinned out 

 that they finally rupture. The membranes may rupture at the 

 same time or each may rupture independently of its mate. 



According to the embryos studied for the substance of this 

 paper the bucconasal membranes rupture from the 35th to the 

 38th day of embryonal life. Sudler found connection between the 

 oral and nasal fossae in a 5-weeks embryo. Hochstetter found in 

 an embryo of ''11 mm. lang bei 9 mm. Kopflange" that the nasal 

 fossae ended blindly dorsally, and in an embryo of ''15.5 mm. 

 Lange und 10 mm. Kopflange," the bucconasal membranes were 

 broken through. He, however, found that the membrane was 

 rather excessive on one side, apparently indicating recent rupture. 

 The approximate age of these embryos would be 33 and 39 days, 



