THE EMBRYOLOGY OF THE OPOSSUM 75 



The second third of the ninth day 



Stage 25. First contact of the neural folds. Fusion of 

 the hearts in the bulbar region. Origin of the sinus venosus. 

 Origin of the pharyngeal floor. The lung anlagen. Miscel- 

 laneous details. Comparative notes. 



First contact of the neural folds. When there are twelve 

 or thirteen pairs of somites the neural folds come together 

 and fuse in the region of the myelencephalon just caudal to 

 the otocysts (fig. 23, A). Almost as soon as the edges fuse, 

 they pinch off from the overlying ectoderm and form a tube 



Fig. 23 Photographs of external views of stage 25. A, dorsal view of 16171 

 by reflected light. B, dorsal view of 16144 by transmitted light. C, ventral 

 view of 16171 by reflected light. 



which is laterally compressed (fig. 25, G and H). During the 

 latter part of stage 25 the tube continues to close caudad 

 through the first six or seven segments, that is, to about the 

 anterior level of the limb ridge (fig. 23, B), but no progress 

 at all is made craniad (fig. 25, A, B, C, D, E and F). In fact 

 the brain plate will not begin to close over until the end of 

 stage 27 (early tenth day, chapter X). 



In the region of closure the neural crest seems to have 

 ceased proliferating, and in the somite region it is aggregated 

 segmentally. The gasserian ganglion and the acousticofacial 

 complex (fig. 24) are rather well demarcated, but are still 

 receiving proliferations from the brain plate. Neural crest 



