SENSORY LAYER 

 PIGMENT LAYER 



BULBUS ARTERIOSUS 

 AORTA 



DIENCEPHALON 



LENS VESICLE 



LARYNGOTRACHEAL GROOVE 



ANTERIOR CARDINAL VEIN 

 ATRIUM 



Fig. 149. Two sections through the lens vesicle, optic cup, and laryngo- 

 tracheal groove. The lens vesicle has separated from the ectoderm and lies within 

 the optic cup. Two layers are distinguishable in the optic cup: (1) a thick 

 sensory layer, which will give rise to the photoreceptors and which contains the 

 neuroblasts for the optic nerve; (2) a pigment layer, which will deposit the 

 black pigment of the eye. From the floor of the pharynx a deep groove develops. 

 This is the laryngotracheal groove, which will form the endodermal lining of 

 lungs and trachea. The sections pass through the bulbus arteriosus and through 

 the atrium of the heart. Note the anterior cardinal veins approaching the atrium. 



SINUS VENOSUS 

 DUCT OF CUVIER 



Fig. 1 50. The olfactory pits. This is a section through the telencephalon and 

 the spinal cord. The olfactory placodes noted at 48 hours invaginate and form 

 pits. Neuroblasts in this tissue will form the olfactory nerve by sending fibers 

 into the cerebral hemispheres. 



In the heart the section passes through the bulbus arteriosus, the atrium, and 

 the sinus venosus. The duct of Cuvier, which receives the cardinal veins, enters 

 the sinus venosus just one section anterior to the one shown here. 



225 



