EXPERIMENTAL FISH EMBRYOLOGY 385 



The eye pigment has become considerably darker and some iridiophores are present. 

 The pupil is ellipsoidal. The olfactory bulbs have completely invaginated. The otic 

 vesicles are enlarged and three crystal-like otoliths are present in each. 



The fin buds are laterally flattened. The caudal tip of the notochord is slightly 

 upturned and the tail tip is laterally compressed, exhibiting a rudimentary sign of a 

 heterocercal type of tail structure. 



A few stellate melanophorea are usually found in the connective tissue above the mid- 

 dorsal, posterior region of the mesencephalon. This is the first indication of extra- 

 ocular melanophores . 



The gut is completely separate from the yolk and the anterior intestinal portion is 

 twisted into two coils. The posterior portion is straight and ends in a somewhat long 

 post-anal region. The gill slits, except the first, are beginning to sink into a common 

 cervical sinus, the forerunner of the opercular cavity. 



Stage 16. Fin Rays; 3.2 mm. 



First indications of fin rays in the caudal and pectoral fins are present. Melano- 

 phores are spreading to the myelencephalon region. 



Stage 17. Anal and Ventral Fins; 3.H mm. 



Anal fin and the skeletal elements of the ventral fins are beginning to appear. 

 Smaller, dot-like, melanophores appear on the lateral body folds. Head la further en- 

 larged and fills the entire pericardial membrane tightly. The operculum is formed at this 

 time. 



Stage 18. Dorsal Fin; 3.7 mm. 



Primordium of dorsal fin becomes visible, but there are no skeletal elements within 

 It. Melanophores have spread over the entire mid- and hind-brain regions. Embryos at this 

 stage are capable of swimming about, although the yolk sac prevents them from rising from 

 the substrate. 



Stage 19. Eyes and Mouth Mobile; 3.9 mm. 



Through the enveloping pericardial membranes, the eyes may be seen to move and the 

 mouth to open. The operculum is functional here. Fascial and peritoneal melanophores ap- 

 pear as small black dots. 



Stage 20. Pericardial Sac Splitting; 1.2 mm. 



The pericardial extra- embryonic membrane begins to split down the dorsal midline, 

 starting at the anterior margin Just above the sinus venosue. (This Is the first step In 

 the formation of the "neck strap," described by Turner (19U0a) in many viviparous 

 cyprinodonts. ) 



Stage 21. Mouth Protrudinn; 1.6 mm. 



The pericardial sac has split open as far as the anterior margin of the eye, allow- 

 ing the mouth to protrude. The peritoneal melanophores are more numerous and small fascial 

 melanophores are concentrated around the notochord. Stellate cutaneous melanophores are 

 very sparsely scattered over the entire embryo, and many are concentrated in the mid- and 

 hind-brain regions. 



