GENERAL GROWTH OF THE EMBRYO. 



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

 invagination from the exterior to form the mouth has appeared 

 (m) on the ventral side of the head close to the base of the 

 thalamencephalon. The upper end of this eventually becomes 

 constricted off as the pituitary body, 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 (at.v), becomes 

 eventually completely 

 atrophied. It is known 

 as the post-anal gut. 

 In the region of the 

 throat the rudiment of 

 a second visceral cleft 

 has appeared behind 

 the first ; neither of 

 them is as yet open to 

 the exterior. 



In a somewhat older 

 embryo the first spon- 

 taneous movements 

 take place, and consist 

 in somewhat rapid ex- 

 cursions of the embryo 

 from side to side, pro- 

 duced by a serpentine 

 motion of the body. 



A ventral flexure 

 of the prae-oral part of 

 the head, known as the 

 cranial flexure, which commenced in earlier stages (fig. 28 D 

 and E), has now become very evident, and the mid-brain 1 begins 

 to project in the same manner as in the embryo fowl on the 



1 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. 



al 



FIG. 28*. FOUR SECTIONS THROUGH THE 

 POST-ANAL PART OF THE TAIL OF AN EMBRYO OF 

 THE SAME AGE AS FIG, 28 F. 



A is the posterior section. 



nc. neural canal ; al. post-anal gut ; alv. caudal 

 vesicle of post-anal gut ; x. sub-notochord rod ; mp. 

 muscle-plate; eh. notochord; cl.al. cloaca; ao. 

 aorta ; v.cau. caudal vein. 



