RECONSTRUCTION MODEL OF A HORSE EMBRYO TWENTY-ONE DAYS OLD. 335 



to the hind-brain curve there is a second curve, convex dorsally, which may be 

 looked upon as the cervical curve ; therefore the three primary flexures of the neural 

 tube are all present : the cephalic flexure, in the mid-brain region, convex dorsally ; 

 the pontine flexure of the hind-brain, concave dorsally ; and the dorsally convex 

 cervical flexure at the region of junction of the hind-brain and the spinal medulla. 



The spinal medulla runs from the hind-brain to the caudal end of the embryo, 

 where it terminates in a mass of cells in which the hind-gut and the notochord 

 also end (text-fig. 23, below). 



Throughout its whole extent the dorsal surface of the spinal medulla lies close to 

 the surface in the dorsal median line, and no mesoderm intervenes between it and 

 the surface ectoderm. 



As the spinal medulla passes caudally it gradually diminishes in size, and at its 



Notochord 



NeuralCanaJ \ Cauda/ Veurvpore 



Spina/ Medulm\ \ ; 



Somatop7e ure \ '. | ! Amnion Cavity 



entoderm 



Text-fig. 23. — Diagram representing a media] sagittal section of the caudal end of the embryo. 



caudal end, where it is not yet separated from the surface ectoderm, its central canal 

 opens on the surface through the caudal neuropore previously mentioned (figs. 54 

 and 59, PI. XIX ; text-fig. 23, above). . 



Nerve ganglia and nerves were present, but no attempt was made to reconstruct 

 them. 



The Otic Vesicle. 



Each otic vesicle is of ovoid form, but somewhat compressed in the cranio-caudal 

 direction. It opens on the surface, at its dorsal apex, between the median plane and 

 the dorsal end of the second visceral arch (fig. 55, PI. XIX). 



It is situated opposite the middle of the concavity of the hind-brain to the medial 

 side of the second branchial pouch, at the level of the cranial border of the pouch 

 (figs. 55, 58, PI. XIX), and very distinctly caudal to the dorsal end of the first 

 branchial pouch. 



The greatest cranio-caudal length of the vesicle, in the embryo, is 150ft. Its 

 immediate relations are : medially, the hind-brain ; ventrally, the corresponding 

 dorsal aorta ; and ventro-laterally, the anterior cardinal vein. 



