THE EMBRYOLOGY OF THE OPOSSUM 113 



small branch of the anterior cerebral shown in the reconstruc- 

 tion (fig. 36) running ventrally at the isthmus, is the trans- 

 verse hypophyseal vein, which crosses the midline and unites 

 the veins of the two sides. 



The first aortic arch has become much more slender, but is 

 still a single channel, not a plexus. The second aortic arch 

 is now the largest ; the third is quite large ; the fourth is com- 

 plete, but small; and the fifth, which in most mammals forms 

 late or not at all, is in process of forming on time (fig. 36). 



The first three gill pouches are well formed and in contact 

 with ectoderm (fig. 36, B). The fourth and fifth are still 

 fused (fig. 33). 



The otocyst is detached from the ectoderm, and the ecto- 

 derm has thickened into a lens placode at the point of contact 

 with the optic vesicle (fig. 36, D). 



There is a slight thyroid depression in the middle of the 

 floor of the pharynx between the second branchial pouches 

 (fig. 33). 



The last third of the tenth day 



Stage 29. External distinctive features. The extra- 

 embryonic membranes. The pharyngeal bursa. The dorsal 

 pancreatic diverticulum. The thyroid anlage. Miscellaneous 

 details. Comparative notes. 



External distinctive features. Stage 29 is distinguished 

 from stage 28 by the naso-oral groove, the tail bud, the club- 

 like enlargement of the end of the anterior limb, the promi- 

 nent posterior limb ridge, the closed or nearly closed amio- 

 pore with some extra-embryonic coelom anterior to its level, 

 and the fact that the omphalomesenteric veins pass behind 

 instead of in front of the anterior limbs. It is distinguished 

 from stage 30 by the absence of the secondary lumbar flex- 

 ure, and by the following negative facts: the anterior limb 

 club has not flattened into a paddle; the posterior limb ridge 

 has not become pointed enough to be called a bud; and the 

 allantois has not enlarged enough to be noticeable externally. 



