226 ^- HATTA; FORMATION OF THE 



Stage II ( Fig. 1 IÎ) : — The head-fold is raised over the general 

 level of the egg, and is protruded a little forwards (at the pole opposite 

 the bhistopore), although, being confined within the chorion, it is 

 necessarily bent backwards and pressed down on the blastoderm. The 

 neural ridge which was slio-htly marked in the fore^'oino- stau'e l^econies 

 prominent along the dorsal longitudinal line. 



Stage III (Fig. 1 C) :— At first sight, this stage is very much like 

 the preceding, l)ut at the tip of the head-end, the dorsal median labial 

 protnl)erance, and just ventral to it, the external mouth pit comes into 

 view ; a few mesoblastic somites and two pairs (the first and the 

 second) of the branchial invaginations can also be seen. 



Stage IV (Fig. 1 D and D') : — When the embryo of this stage is 

 widiiii the eg(y memljrane, it is twisted in a peculiar manner, in 

 consequence of the enormous prolongation of the head-fold, so tliat the 

 head rests on the expanded yolk-sac ; three pairs of the gill-in vagi na- 

 tions are visible. If the kigg membrane is broken about the end oï this 

 stiige, the embryo straightens and becomes retort-shnped (Fig. IE.) 

 It measures al)out '2.b mm. and has from three to four pairs of the 

 gill-in vaginations. Corresponding with the external development, 

 many profound internal changes take place, such as the first apip'carance 

 of the pronephros, (jf the heart, of the liver, and m forth. 



Stage Y (Fig. IF) : — In this stnge the embryo hatches spontaneous- 

 ly and is pistol-shaped ; it measures about o mm. with five pairs of 

 the branchial invaginations. 



Stage VI (Fig. 1 G) : — The tail-end begins to grow and to point 

 out, so that tlie yolk-expansion lies between the head-fold and the 

 tail-protuberance ; it measures about 5 mm. and has from six t(3 seven 

 pairs of the gills. In this stage the gill-pouches begin to (3pen out- 

 wards, and before it is over, all pairs of the persistent gills acc^uire 

 free communication with the exterior. 



