ELASMOBRANCHII. 59 



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 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 

 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 

 before, 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 appear is very 

 easily detected by a bulging out of the gut towards the external 

 skin. The alimentary vesicle at the end of the post-anal gut, 

 first observable during the last stage, is now a more conspicuous 

 organ. There are three visceral clefts, none of which are as yet 

 open to the exterior. 



Figure 28 F represents a considerably older embryo viewed 

 as an opaque object, and fig. 29 A is a view of the head as a 

 transparent object. The stalk connecting it with the yolk is 

 now, comparatively speaking, quite narrow, and is of sufficient 

 length to permit the embryo to execute considerable move- 

 ments. 



The tail has grown immensely, but is still dilated terminally. 

 The terminal dilatation is mainly due to the alimentary vesicle 

 (fig. 28* alv), but the post-anal section of the alimentary tract in 

 front of this is now a solid cord of cells. Both the alimentary 

 vesicle and this cord very soon disappear. Their relations are 

 shewn in section in fig. 28*. 



The two pairs of limbs have appeared as differentiations of a 

 continuous but not very conspicuous epiblastic thickening, which 

 is probably the rudiment of a lateral fin. The anterior pair is 

 situated just at the front end of the umbilical stalk ; and the 



