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YVETTE W. KUNZ 



inner sheet of the latéral folds is made up of 1-2 layers of cuboidal cells, whereas 

 the outer sheet is composed of flattened cells. This indicates that the thick 

 embryonic ectoderm and the thin extra-embryonic ectoderm equally contribute 

 to the folding process. In the ventral région of the fused folds the layers are 

 pressed between brain and yolk and consequently ail their cells appear to be 

 greatly flattened (Fig. 6a, 7a). 



Stage 4 (6th embryonic day) 



a) Lens of eye appears. A vascular network develops over the surface of 

 the yolk sac. 9-16 somites présent (Fig. 4a). 



b) The lens placode, a solid spherical mass of cells, fully séparâtes from 

 the overlying superficial nervous ectoderm. The optic vesicles begin to inva- 

 ginate into optic cups. Infundibular région of diencephalon is deep and distinct 

 and the third ventricle becomes established. Olfactory organs appear as a pair 

 of solid cell masses. Auditory placodes acquired a lumen and become the otic 

 vesicles (Fig. 6b). 



The bilatéral pericardial primordia meet ventro-medially and form the endo- 

 cardial tube. They greatly extend in the anterior direction and towards the end 

 of this stage, extend beyond the head. There is, as yet, no indication of a peri- 

 cardial overgrowth of the head (Fig. 6b, 7b). Ectodermal headfolds extend 

 longitudinally to the posterior région of the mesencephalon, near the first gill 

 pouch. The layers of the ectodermal headfold are now greatly thinned consisting 

 of flattened cells which no longer reveal their origin as of embryonic or extra- 

 embryonic nervous ectoderm. 



Stage 5 (lOth embryonic day) 



a) Average length of embryo 3.6 mm. Eyes are pigmented grey. As a 

 resuit of cranial flexure, the head becomes further buried in the yolk, which 

 now reaches to the upper level of the eyes. The urinary bladder, dorsal to rectum, 

 is clearly visible. Tail develops and primordia of the caudal and anal fins are 

 évident. Pectoral fin buds appear. Blood leaves the embryo through the vena 

 caudalis, ductus Cuvieri and vena hepatica. It flows over the yolk sac in ventral 

 and anterior directions, and enters the heart at an expanded portion, the future 

 sinus venosus, which lies considerably anterior to the head (Fig. 4b). 



b) The eyes possess a thin pigment layer and the lens is now fibrillar. Optic 

 lobes begin to form. Otic vesicles contain otoliths (Fig. 6c). 



The bilatéral pericardium has formed folds which extend dorsally, and at 

 the same time push the bases of the ectodermal headfolds until they meet in the 

 mid-dorsal line. This mid-dorsal line is marked by a "somatopleural bridge", 

 which consista of two layers of nervous ectoderm (the two displaced bases of the 

 headfolds) and two layers of the pericardial mesothelium. Posteriorly, the peri-j 



