124 



FISHERY BULLETIN OF THE FISH AKD WILDLIFE SERVICE 



Nostril: Begins as single invagination in mid- 

 ventral line, anterior to the stomodaeum. 



Transparency: First appearance of ectodermal 

 transparency over pericardinm. Extends pos- 

 teriorly and anteriorlj' in later phases of this stage. 



Circulatory system: Pericardium visible through 

 transparent ectoderm. Straight tubular heart. 

 Begins beating 40 times per minute. 



Liver: First indication posterior to pei'icardium 

 in anterior part of gut. Appears in late period of 

 stage. 



Stage 14 starts when the embryo breaks through 

 the fertilization membrane. The constant move- 

 ment observed in stage 13 finally becomes suffi- 

 ciently strong to extrude the head through the 

 membrane. Further activity enlarges the tear in 

 the membrane and eventually leads to hatching. 



The pericardial cavity appears as a ventral swell- 

 ing approximately 1.0 millimeter posterior to the 

 tip of the head. It is also present in late stage 13 

 but is not easily recognized in most embryos. 

 Shortly after hatching the ectoderm overlying the 

 pericardial region and the tissue beneath gradually 

 become transparent. A short time later, the body 

 anterior to the pericardial cavity becomes trans- 

 parent. 



Through these transparent tissues can be seen 

 the straight tubular heart which begins to pulsate 

 during the eleventh day at 40 beats per minute (at 

 65° F.). There is no sign of blood in the heart or 

 anywhere in the prolarva. 



The end-point of stage 14 is reached when 

 melanophores appear on the embryo. 



Stage 15: Pigmentation (fig. 22) days 13-16 



Size: 5 to 6 millimeters. 



Neural tissue: Brain and tube visible through 

 ectoderm. 



Gut: Spatulate, changing to cylindrical. Ven- 

 tral flexion of about 10° remains. Anterior face 

 greenish. 



Stomodaeum (mouth) : Transverse sUt bounded 

 by thickened lips. Opens into oral cavity. 



Somites: 35 to 50. 



Locomotion: Undulation of entire body slightly 

 restricted by yolk-filled gut in early stage. Full 

 swimming movements in late period. 



Nostril: Single, median at anteriormost point on 

 ventral surface. 



Transparency: Extends anteriorly to branchial 

 region and posteriorly to about two-thirds the 

 length of the prolarva. 



Circulatory system: Heart becomes S-shaped. 

 Heart walls thicken. Grayish channel forms in 

 midventral gut and turns red as hemoglobin ap- 

 pears. Bilateral channels appear in 15th day. 

 Heart beat, 100 per minute. 



Liver: Becomes larger and vascularized. 



Pigmentation: First appears as bilateral melano- 

 phores dorsal to the midbrain. 



The stomodaeal pit has become a slit opening 

 into the anterior (oral) chamber of the pharynx 

 which is separated from the posterior (pharyngeal) 

 chamber by the velum. The transverse slit is 

 bounded by a thickened ectodermal lip anteriorly 

 and posteriorly. The pharynx has its full comple- 

 ment of 7 visceral pouches. 



Deftness of swimming increases as the shape of 

 the yolk mass changes from spatulate to cylindri- 

 cal. At the same time the tail straightens from its 

 ventral flexion. The prolarvae progress from the 

 awkward movements of stage 14 to an undulating 

 movement, the smoothness of which depends on 

 the amount of the ventral flexion of the tail. 



The heart has enlarged and assumes an S-shape 



Figure 22. — Lateral view of stage 15, pigmentation, showing somites, pericardial area, and condition of yolk. 



