GENERAL FEATURES. 29 1 



The fore and hind guts are now longer than they were. A 

 slight pushing in from the exterior to form the mouth has 

 appeared (m.~), and an indication of the future position of the 

 anus is afforded by a slight diverticulum of the hind gut towards 

 the exterior some little distance from the posterior end of the 

 embryo (an.}. The portion of the alimentary canal behind this 

 point, though at this stage large, and even dilated into a vesicle 

 at its posterior end (al. v.}, becomes eventually completely 

 atrophied. In the region of the throat the rudiment of a second 

 visceral cleft has appeared behind the first ; neither of them are 

 as yet open to the exterior. The number of visceral clefts 

 present in any given Pristiurus embryo affords a very easy and 

 simple way of determining its age. 



I. 



A great increase in size is again to be noticed in the embryo, 

 but, as in the case of the last embryo, it has not been possible to 

 represent this in the figure. The stalk connecting the embryo 

 with the yolk has become narrower and more elongated, and 

 the tail region of the embryo proportionately far longer than in 

 the last stage. During this stage the first spontaneous move- 

 ments of the embryo take place, and consist in somewhat rapid 

 excursions of the embryo from side to side, produced by a 

 serpentine motion of the body. 



The cranial flexure, which commenced in stage G, has now 

 become very evident, and the mid-brain 1 begins to project in the 

 same manner as in the embryo fowl on the third day, and will 

 soon form the anterior termination of the long axis of the 

 embryo. The fore-brain has increased in size and distinctness, 

 and the anterior part of it may now be looked on as the unpaired 

 rudiment of the cerebral hemispheres. 



Further growths have taken place in the organs of sense, 

 especially in the eye, in which the involution for the lens has 

 made considerable progress. The number of the muscle-plates 

 has again increased, but there is still a region of unsegmented 



] The part of the brain which I have here called mid-brain, and which unquestion- 

 ably corresponds to the part called mid-brain in the embryos of higher vertebrates, 

 becomes in the adult what Miklucho-Maclay and Gegenbaur called the vesicle of 

 the third ventricle or thalamencephalon. I shall always speak of it as the mid-brain. 



