FISH FOR EXPERIMENTATION 373 



Stage 17. Anal and Ventral Fins; 3. 4 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 is further enlarged and 

 fills the entire pericardial membrane tightly. The operculum is forn:ied 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 ris- 

 ing from the substrate. 



Stage 19. Eyes and Mouth Mobile; 3. 9 rnm. 



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 

 appear as small black dots. 



Stage 20. Pericardial Sac Splitting; 4. 2 mm. 



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

 starting at the anterior margin just above the sinus venosus. (This is the first step in 

 the formation of the "neck strap, " described by Turner (1940a) in many viviparous 



cyprinodonts. ) 



Stage 21. Mouth Protruding; 4. 6 mm. 



The pericardial sac has split open as far as the anterior margin of the eye, allowing 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. 



Stage 22. Broad "Neck Strap"; 5. 1 mm. (Fig. 20) 



The pericardial membrane has split as far back as the posterior third of the eye, ex- 

 hibiting a broad, vascularized "neck strap". The appearance of sclerotomes is here ac- 

 centuated by concentrations of small melanophores in the fascial tissue around them. 

 Cutaneous melanophores are more numerous, sometimes present in the caudal fin rays. 



Stage 23. Fin Rays in Dorsal Fin; 6. 1 mm. 



The "neck strap" (pericardial membrane) has been reduced to about one-half the width 

 of the eye, and is situated back of the posterior margin of the eye. Fin rays begin to 

 appear in the dorsal fin. Large cutaneous melanophores are thickly scattered over the 

 entire embryo. The yolk sac begins to show a rapid reduction in size, nneasuring 1 mm. 

 in diameter. It is noteworthy that the yolk sac begins to involute at about the same time 

 that the pericardial membranes are in the process of accelerated regression. 



Embryos at this stage, if removed from their mothers, will feed readily on small Daphnia. 



Stage 24. "Neck Strap" Breaking Down; 6. 5 mm. 



The "neck strap" may be completely broken down at this stage, but it is sometimes 

 present as a narrow band of tissue. The general shape of the embryo is determined by 

 the condition of the "neck strap, " the cephalic flexure straightening as the head lifts up 

 into the main body axis. The melanophores in the dorsal head region are stellate and 

 more closely packed. 



