EXPERIMENTAL FISH EMBRYOLOGY 383 



from a solid core of invaginating tissue; the neurocoele appearing after Invagination is 

 completed, as seen in sectioned material. This type of neurulation is typical of teleosts. 



Stage 7. Late Neurula (Fig. 7) 



The germ ring at this stage is somewhat helow the equator and the embryo has hecome 

 further elongated. Since elongation takes place principally in the posterior portion, a 

 region roi;ighly corresponding to the dorsal hlastopore lip of amphibian gastrulae, the 

 anterior end of the embryo lies in much the same position as did the original emijryonic 

 shild of stage 5. . 



The neural keel has invaginated throughout the greater length of the embryo, and a 

 neurocoele is present in the anterior one-fourth. 



Stage 8. Head Fold (Fig. 8) 



A prominent head fold is present by stage 8. The neurocoele is open for about the 

 anterior half of the length of the embryo. The optic buds are present and attached to the 

 short, thin stalks, and they are, at this stage, without a cavity. Two pairs of rather 

 diffuse somites are evident, but there is considerable variation in the time of their 

 first appearance. Somites sometimes appear as early as stage 7- 



Stage 9. Optic and Otic Vesicles; I.I mm. (Fig. 9) 



The head fold has now begun to elongate anteriorly. The blastopore is still a wide 

 open structure and the caudal end has not progressed back much farther than its position 

 in stage 8. The optic prlmordla now possess cavities, and are usually still attached to 

 the prosencephalon by thin optic steilks. The brain is divided into three general regions: 

 a narrow prosencephalon, a slightly wider mesencephalon, and a short rhombencephalon. 

 Otic vesicles have Invaglnated at the level of the rhombencephalon, but are still connected 

 to the exterior by the endolymphatic ducts. Usually, 7 pairs of somites are visible at 

 this stage. 



The pericardial sac, which develops very early, closely enfolds most of the head fold 

 at this stage. 



Stage 10. Tail Gud; 1.5 mm. (Fig. 10) 



The optic vesicles are detached from the brain and are slightly flattened around the 

 invaginating lens prlmordla. The mesenchephalon and rhombencephalon have become widened 

 and more thin walled. The otic vesicles are slightly ellipsoid and are completely cut off 

 from the superficial ectoderm. There are ten pairs of compact somites visible. The tail 

 bud has begun to form and extends slightly over the region of the dorsal lip of the open 

 blastopore. 



The region of the pericardial sac that is extra-embryonic is easily distinguishable, 

 and, upon dissection, the heart can be found as a straight tube on the floor of the peri- 

 cardial sac. The vascular system is apparently complete at this time, but the blood is- 

 lands are never visible under gross examination. The heart exhibits no regular beat, only 

 an occasional twitch. 



Stage II. Pectoral Fin Buds (Fig. II) 



The optic vesicles partially envelop the lens prlmordla. The prosencephalon shows 

 little differentiation, but the mesencephalon has widened out considerably. Indications 

 of neuromeres can be seen In the rhombencephalon. The entire brain possesses a thin roof, 

 and this is especially true at the hind-brain level. In later stages, the roof of the 

 mesencephalon becomes thickened, but that of the i^yelencephalon remains thin as the pos- 

 terior tela chorloldea. The otic vesicles show little or no change, aside from a general 

 growth, in this and several of the following stages. Fig. 11 shows the presence of the 

 anterior fin buds. 



