372 FISH FOR EXPERIMENTATION 



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 pharyngeal 

 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 max- 

 imal size. In the future stages the head enlarges to fill the serosa-like 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 Motility; 2.8 mm. (Figs. 16, 17) 



The head is expanded to almost 0. 5 mm. across the mesencephalon. The eyes exhibit 

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

 sesses 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 typical of the teleosts. Both the metencephalon, which is poorly defined, and the 

 myelencephalon 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 

 myotomes, and, when the living embryo is removed from its membranes, the posterior 

 portion 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 toward 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 di- 

 encephalon is shorter than the telencephalon and less distinct. The optic lobes possess 

 a solid roof. The metencephalon is more distinct and thickened, and the myelencephalon 

 is somewhat narrowed. 



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

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

 icles 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 melanophores 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. Melanophores 

 are spreading to the myelencephalon region. 



