THE EMBRYOLOGY OF THE OPOSSUM 123 



at a very early time, whereas the stomach is not needed ex- 

 cept as a reservoir and has no glands at birth. 



The thyroid anlage. The thyroid in stage 29 is repre- 

 sented by a median ventral pocket in the floor of the pharynx 

 between the first and second branchial pouches, but more 

 nearly in line with the second pair (fig. 33). This location is 

 just cranial to the truncus arteriosus. There has been a slight 

 depression at this point since late stage 27, but it becomes 

 quite distinct in stage 29, and in late specimens is somewhat 

 constricted at the neck. 



Miscellaneous details. The pharyngeal membrane breaks 

 down and is resorbed during stage 29. The trachea sepa- 

 rates from the oesophagus by the fusion and resorption of 

 the middle portion of the lateral walls from behind forward 

 (fig. 33). 



The cranial ganglia send processes into the central nervous 

 system. The posterior neuropore is still open on the tail bud 

 (fig. 39, I and L). The otocyst acquires an endolymphatic 

 duct (fig. 63, e.d.) which grows out from the dorsomedial tip 

 of a vertical ridge on the medial wall. This point of origin 

 is medial to the seam of closure, which is still recognizable 

 at s.c. The first suggestion of demarcation of the seventh 

 (geniculate) ganglion from the acoustic is recognizable in 

 their most ventral parts, but the separation is imperfect and 

 irregular. Dendrites from the eighth ganglion reach the oto- 

 cyst. The lens placode is depressed and is in close contact 

 with the primary optic vesicle. 



The first aortic arch breaks down, leaving that portion of 

 the ventral aorta which connected it with the second arch 

 as the external carotid artery supplying the mandibular arch. 

 The fifth aortic arch is still incomplete. Posterior to the level 

 of the fifth arch a plexus from the ventral aorta extends 

 caudad along the side of the trachea. The vitelline veins ap- 

 proach each other very closely in the midline behind the pan- 

 creatic diverticulum on the dorsal side of the foregut. Here 

 they establish capillary communication with each other. In 

 late specimens a similar situation has developed ventrad of 

 the gut and caudad of the liver bud. 



