RECONSTRUCTION MODEL OF A HORSE EMBRYO TWENTY-ONE DAYS OLD. 333 



folded upon itself, but neither the constriction nor the folding appears to be due to 

 any external extra-embryonic cause. 



An elevated ridge which marks the position of the neural tube runs along the 

 median part of the dorsal region of the caudal portion of the embryo (fig. 54, 

 PL XIX). 



At the caudal end of the median ridge there is a narrow cleft, 100/a long in the 

 embryo, which communicates with the central canal of the neural tube (figs. 54, 61, 

 PI. XIX, and text-fig. 22, p. 334), and constitutes the caudal neuropore. 



In the cranial part of the caudal portion of the embryo there is, on each side of 

 the median neural elevation ridge, an elevation which indicates the position of the 

 subjacent pleuro-pericardial canal (figs. 55, 56, PL XIX). 



When the model is examined from below, the dorsal surface of the cranial portion 

 and the ventral surface of the more caudal part of the caudal portion of the embryo 

 are seen (fig. 55, PI. XIX). In the median line of the cranial portion the neural 

 ridge, already noted on the caudal portion, extends cranially to the level of the 

 dorsal end of the fourth branchial arch. There it terminates in a concavity which 

 corresponds in position with the dorsal wall of the hind-brain (figs. 55, 57, 58, 

 PI. XIX). 



Still more cranially is the convexity of the mid-brain ; at that point the head 

 bends towards the ventral surface of the embryo and it terminates in a rounded con- 

 vexity, which forms the cranial boundary of the stomatodseum and which is bulged, 

 on each side, by the projection of the rudimentary optic vesicle (fig. 57, PI. XIX, 

 and fig. 36, PI. XV). There is no anterior neuropore. 



The portion of the caudal part of the embryo which is seen when the model is 

 viewed from below is a slightly concave and smooth area which forms a part of the 

 wall of the yolk-sac ; for although the area in question will eventually form part of 

 the wall of the alimentary canal, the separation of the post-umbilical part of the 

 alimentary canal from the yolk-sac has not yet commenced, except at the caudal end 

 of the embryo ; there a narrow, antero-posterior, cleft indicates the position of the 

 already partially enclosed cloacal portion of the hind-gut (fig. 55, PI. XIX). 



When the model is examined from the side (figs. 56, 58, PI. XIX) the coelomic 

 space is very obvious in the caudal portion of the embryo, where it intervenes between 

 the splanchnopleure and the somatopleure, and the peculiar folding of the somato- 

 pleure, immediately before it becomes continuous with the amnion, is very noticeable. 

 It is most obvious in the constricted region which was noted in the description of 

 the dorsal aspect of the caudal portion (fig. 54, PI. XIX). 



On each side of the cranial portion of the embryo four branchial arches, with three 

 intervening furrows, are easily recognised. 



The ventral ends of the first arches are fused together at the cranial border of 

 the pericardial region, which they separate from the stomatodseum ; but the ventral 

 ends of the three more caudally situated arches terminate on the dorsal border of 



