THE EMBRYOLOGY OF THE OPOSSUM 91 



But what about the early development of the ear? The 

 opossum has no functional vestibular apparatus until about 

 41 days after birth, and cannot hear until about the fiftieth 

 day. So there would seem to be no need for an early start 

 in the differentiation of the otocyst. 



I believe the explanation lies in the fact shown by a number 

 of operative experiments on amphibians that the otocyst 

 is induced by the underlying tissues. The hyoid mesoderm, 

 according to Harrison ('35), is the primary inductor, and the 

 hind-brain a secondary inductor. If the inductor is pre- 

 cocious, the induced organ will be precocious. We have al- 

 ready seen that both of these inductors are precocious for 

 other reasons. Paradoxical as it may sound, the ear develops 

 early, not because the opossum will have to hear at an early 

 stage, but because he will have to breathe, and suck, and use 

 his stomach at an early stage. 



X. THE TENTH DAY. Stages 27 to 29 



The first third of the tenth day 



Stage 27. External distinctive features. The cervical 

 flexure. The lung buds. The tail fold of the anmion. 

 Changes in the vascular system. Miscellaneous details. 



External distinctive features. Stage 27 is distinguished 

 externally from stage 26 by the cervical flexure and the tail 

 fold of the amnion ; from stage 28 by the fact that the anterior 

 neuropore and the otocysts are still wide open. 



Associated with the cephalic and cervical flexures are 

 changes in the extent of the head fold of the amnion. In 

 stage 26 the cephalic flexure had occurred in the region of the 

 midbrain and the head had become enclosed in the pro-amnion 

 back to the level of the otocysts. In stage 27 the cervical 

 flexure occurs at the level of the cervical cord, the heart, and 

 the first somites, and the head fold passes over the otocysts 

 and back to this new level. The caudal fold, which will be de- 

 scribed below, first appears in stage 27 and covers part or all 

 of the sinus rhomboidalis. 



