Mervin T. Sudler 407 



It is only when an embryo of alDOut -i^ weeks old (embryo CLXIII) 

 is studied that the olfactory organ has taken on the form of pits. Here 

 they are two bean-shaped cavities (see Figs. 4 and 6). The candal end 

 is enlarged and a shallow groove runs towards the mouth cavity. It is 

 not yet connected with the mouth cavity so it is not shown in Fig. 4 

 which is draAvn from the side. The ridge overlying it, however, is 

 shown faintly outlined. At its outer angle the superior maxillary 

 process stands out as a rounded eminence. Fig. 6 shows the frontal 

 jDrocess and an idea can be gotten of the comparative distance of the 

 olfactory pits from the middle line in an embryo of this stage. In an 

 older stage (5 weeks), represented by the model of embryo CIX, the 

 four parts mentioned by His as forming the nose and mouth have 

 united. In Fig. 7, which shows this embryo from the side, the nasal 

 cavity is represented by two flattened processes attached to the buccal 

 cavity by two slender stalks. These meet the mouth at an angle; that 

 is, they diverge slightly as they leave the mouth cavity. These two 

 flattened surfaces bear suggestions of ridges which are developed in 

 the later stages, the most prominent being a cephalic dorsal one run- 

 ning backward toward the mouth opening. Seen from the inside there 

 is a decided prominence at the angle Avhere the pedicle and main nasal 

 cavity join one another. This is the beginning of Jacobson's organ. 



In an embryo slightly older, such as we have in the model of embryo 

 CVI (this model is not figured), the plates seen from the side are no 

 longer flat and the ridges shown in embryo CIX have become developed 

 to so much greater extent that they can be distinctly recognized. 

 These probably mark the first divisions into ridges and furrows that 

 mark the rudiments of the turbinate processes. On the inner side the 

 surface is also flattened and there is a small rounded knob marking 

 the position of Jacobson's organ as mentioned for embryo CIX. Con- 

 sidering this part of the nasal cavity as a whole, the model shows it 

 to be decidedly three sided. A cross-section of it would represent a 

 right angled triangle. The right angle being at the junction of the 

 outer surface with the lower (caudal) surface. The inner surface 

 slopes downward towards the middle line and completes the outline 

 of the triangle. The anterior nares run up to join this structure as 

 a rounded body and meet it at right angles to the remainder of the 

 structure. The posterior nares are a little larger than the anterior 

 and show on their outer dorsal surfaces ridges continuous with the 

 ridges on the surface of the nasal cavity. The entire model looked at 

 from above shows that there is no angle of divergence between the 

 models of the nasal cavities. They meet the mouth at almost right 

 28 



