THE EMBRYOLOGY OF THE OPOSSUM 93 



lift away from the pharynx at the level of the lung buds 

 (fig. 30, reconstruction and sections B and C). This is the 

 cervical flexure. Both it and the cephalic flexure are con- 

 spicuously reflected in the external body form. 



The lung buds. The earliest anlagen of the lungs were men- 

 tioned in chapter IX where they were pointed out in stages 

 24 and 25 as first, the mesothelial thickenings, and then, as- 

 sociated local endodermal outpocketings from paired longi- 

 tudinal branchial furrows. In figure 31 these furrows can be 

 followed to the point where the lung pouches are located at 

 the dorso-lateral corners of the pharynx, which is just in 

 process of acquiring a floor at their level (stage 27). Atten- 

 tion has already been called to the striking similarity between 

 the lung pouches of stage 25 and the second branchial pouches 

 of stage 24-)-. 



Figure 32 shows photographs of models of the pharynx 

 taken from early and late specimens of stage 27. The early 

 specimen (A) shows a pharynx which is triangular in section 

 at the lung level, with the apex placed ventrally between the 

 venous sinuses, and the lungs situated at the dorso-lateral 

 corners of the triangle. The roof of the pharynx is beginning 

 to lift into a dorsal ridge which forms when the central 

 nervous system and the notochord lift dorsally as just de- 

 scribed in connection with the cervical flexure. The late speci- 

 men (B) shows the ventral apex obliterated (by the con- 

 crescence of the atria of the hearts) and the dorsal ridge lifted 

 so high that the lung buds are now at the ventro-lateral 

 corners of a new triangle. 



These changes are represented from the lateral view in 

 figure 33. The branchial ridge illustrated here is, of course, 

 the outside view of the branchial furrow previously referred 

 to. The rise in the floor of the pharynx beneath the first 

 branchial pouches in stage 25 is due to the fusion of the an- 

 terior end of the heart tubes into a single median bulbus 

 cordis. The ridge labeled dorso-mesocardial line indicates 

 the position of the mesocardium of the atrium, which does 

 not meet its fellow of the opposite side in the midline until 

 the end of stage 27. 



