53U EXPERIMENTAL FISH EMBRYOLOGY 



Posteriorly, ib to 20 small, compact somite pairs blend into a poorly differentiated 

 region in the now prominent tail bud. It is noteworthy that, although a sizeable tail bud 

 is present at this stage, the blastopore is open in majority of the embryos. This is in 

 contrast to the case in most teleosts, and even in the closely related Fundulus. 



In the heart, the ventricular and atrial portions are distinct, and at the anterior 

 end, the sinus venosus projects In front of the head. The heart exhibits a fairly rhyth- 

 mical beat at this time. The color of the blood is light pink, but barely perceptible. 



Stage 12. Regular Heart Beat; 1.8mm. (Figs. 12, 13) 



The optic cupa envelop the lenses closely. Olfactory placodes are visible. The brain 

 has undergone further development; the telencephalic region is slightly expanded; the mes- 

 encephalon has a thicker roof; the rhombencephalon is greatly widened. 



The somites are more closely packed and less distinct. The vascularization of the 

 pericardial membrane is in the form of small capillary-size vessels. The extra -embryonic 

 circulation can be followed at this stage. The blood leaves the embryo through the ducts 

 of Cuvier at the posterior ventral margin of the pericardial membrane, drains into the 

 yolk portal system and the vascularized pericardial membrane, and collects at the elongated 

 sinus venosus. 



The mid-gut is broad and extends under about one-third of the embryo. The hind-gut 

 is short, and the fore-gut, upon dissection, is shown to possess a distinct first pharyn- 

 geal pouch and a corresponding visceral furrow. 



Stage 13. Early Retinal Pigment; 2.1 mm. (Figs. 14, 15) 



Olfactory pits are distinct. Pigment can be seen in the retina as a thin gray band. 

 The brain and the head are further enlarged. The pericardial sac has increased to its 

 maximal size. In the future stages the head enlarges to fill the serosa-llke cavity and 

 sinks down into the yolk mass. In side view, the stomodaeum, five gill clefts and the 

 sixth furrow can be seen. 



Stage 14. Early Motilitv; 2.8 mm. (Figs. 16, 17) 



The head is expanded to almost 0.5 nun- across the mesencephalon. The eyes exhibit 

 more pigment and are pushed forward by the expanding mesencephalon. The latter possesses 

 a thickened roof where the optic lobes are developing. The telencephalon has a somewhat 

 rhomboidal- shaped cavity and the diencephalon is small and hardly distinct; this is typi- 

 cal of the teleosts. Both the metencephalon, which is poorly defined, and the nyelen- 

 cephalon have thin roofs. The neuromeres are still visible in the latter. 



The heart possesses a long sinus venosus and a narrow atrium that has been twisted to 

 the left of the thick-walled ventricle. The blood vessels of the pericardial membrane are 

 enlarged to a size equal to almost one-half the diameter of the ducts of Cuvier. 



The anterior fin-buds are club-shaped and rounded. The somites have taken the form 

 of nyotomes, and, when the living embryo is removed from its membranes, the posterior por- 

 tion exhibits a slow twitching motion. The tail is conical and acuminate. 



All six gill slits are distinct and open at this stage. The mid-gut is narrowed to- 

 ward the posterior portion of the embryo, and fore-gut is an undifferentiated tube. 



Stage 15. Otoliths in Ear Vesicles; First Extra-ocular 

 Melanophores; 3.2 mm. (Figs. 18, 19) 



In this stage the telencephalic vesicles are beginning to show as lateral bulges. 

 The diencephalon Is shorter than the telencephalon and less distinct. The optic lobes 

 possess a solid roof. The metencephalon is more distinct and thickened, and the nyelen- 

 cephalon is somewhat narrowed. 



