48 



GENERAL GROWTH OF THE EMBRYO. 



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 spontaneous movements 

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

 from side to side, produced by a serpentine motion of the body. 



A ventral flexure of the prseoral 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^ 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 rudi- 

 ment of the cerebral hemispheres. 



Further changes 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 mesoblast in the tail. The 

 thickened portions of mesoblast, which caused the tail swellings, are 



still to be seen, and would 

 ■^ B seem to act as the reserve 



from which is drawn the 

 matter for the rapid growth 

 of the tail, which occurs 

 soon after this. The mass 

 of the mesoblast at the 

 base of the brain has 

 again increased. No fresh 

 features of interest are to 

 be seen in the notochord. 

 The heart is very much 

 more conspicuous than be- 

 fore, and its commencing 

 flexure is very apparent. 

 It now beats actively. 

 The post-anal gut is much 

 longer than during the 

 last stage ; and the point 

 where the anus will ap- 

 pear is very easilydetected 

 by a bulging out of the 

 gut towards the external 

 skin. The alimentary 

 vesicle at the end of the 

 post-anal gut, first observ- 



FlG. 28*. FOUE 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. subnotochord rod; vip. 

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

 aorta; v.cau. caudal vein. 



^ 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 Milducho-Maclay and Gegenbaur called the vesicle of the 

 third ventricle or thalamencephalon. 



