DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 in- 

 creased. No fresh features of interest are to be seen in the 

 notochord. The heart is now much more conspicuous than 

 before, and its commencing flexure is very apparent. It now 

 beats actively. The hind gut especially is much longer than 

 in the last specimen ; and the point where the anus will appear 

 is very easily detected by the bulging out of the gut towards 

 the external skin at that point (an.}. The alimentary vesicle, 

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

 organ (al. v.). Three visceral clefts, none of which are as yet 

 open to the exterior, may now be seen. 



K. 



The figures G, H, I are representations of living and trans- 

 parent embryos, but the remainder of the figures are drawings of 

 opaque embryos which were hardened in chromic acid. 



The stalk connecting the embryo with the yolk is now, com- 

 paratively speaking, quite narrow, and is of sufficient length to 

 permit the embryo to execute considerable movements. 



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

 This terminal dilatation is mainly due to the alimentary vesicle, 

 but the tract of gut connecting this with the gut in front of the 

 anus is now a solid rod of cells and very soon becomes com- 

 pletely atrophied. 



The two pairs of limbs have appeared as elongated ridges 

 of epiblast. The anterior pair is situated .just at the front 

 end of the umbilical stalk ; and the posterior pair, which is 

 the more conspicuous of the two, is situated some little distance 

 behind the stalk. 



The cranial flexure has greatly increased, and the angle 

 between the long axis of the front part of the head and of the 

 body is less than a right angle. The conspicuous mid-brain 

 forms the anterior termination of the long axis of the body. 

 The thin roof of the fourth ventricle may in the figure be noticed 

 behind the mid-brain. The auditory sac is nearly closed and its 



