DEYELOPMEXT OF ELAS^IOBP.AXCH FISHES. 77 



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 un- 

 segmented 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 

 asfain increased. 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 Avill 

 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 stasre, 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, wdiich 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 



