FISH FOR EXPERUVlENTATION 371 



Stage 9- Optic and Otic Vesicles; 1. 1 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 primordia now possess cavities, and are usually still attached to the 

 prosencephalon by thin optic stalks. The brain is divided into three general regions: a 

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

 Otic vesicles have invaginated 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 Bud; 1.5 mm. (Fig. 10) 



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

 vaginating lens primordia. The mesencephalon 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 pericardial 

 sac. The vascular system is apparently complete at this time, but the blood islands are 

 never visible under gross examination. The heart exhibits no regular beat, only an occa- 

 sional twitch. 



Stage 11. Pectoral Fin Buds (Fig. 11 ) 



The optic vesicles partially envelop the lens primordia. 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 myelencephalon remains thin as the 

 posterior tela chorioidea. The otic vesicles show little or no change, aside from a gen- 

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

 the anterior fin buds. 



Posteriorly, 18 to 20 small, compact somite pairs blend into a poorly differentiated re- 

 gion 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 con- 

 trast 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 rhythmical beat 

 at this tiiTie. The color of the blood is light pink, but barely perceptible. 



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



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

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

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



The somites are nnore closely packed and less distinct. The vascularization of the peri- 

 cardial 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. 



