76 GENERAL FEATURES. 



still a certain mass of unsegmented mesoblast whicli forms 

 the tail-swellings. The first rudiment of the heart becomes 

 visible during this stage as a cavity between the mesoblast 

 of the splanchnopleure and the hypoblast (ht.). 



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 alimxcntary canal 

 behind this point, though at this stage large, and even dilated 

 into a vesicle at its posterior end (alv.), becomes eventually 

 completely atrophied. In the region of the throat the rudi- 

 ment 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 con- 

 necting the embryo with the yolk has become narrower and 

 more elongated, and the tail region of the embryo propor- 

 tionately far longer than in the last stage. During this stage 

 the first spontaneous movements 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^ 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 dis- 

 tinctness, and the anterior part of it may now be looked on 

 as the impaired rudiment of the cerebral hemispheres. 



Further growths have taken place in the organs of sense, 



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

 unquestionably corresponds to the part called mid-brain in the embryos of higher 

 vertebrates, becomes in the adult what Miklucho-IMaclay and Gegenbaur called 

 the vesicle of the thii'd ventricle or thalamencephalon. I shall always speak 

 of it as the mid-brain. 



