490 EMBRYOLOGY OF THE CHICK 



also empties into the left vitelline vein and carries blood from the pos- 

 terior region of the sinus terminalis, makes its appearance. 



It will be remembered that it is during this day that the torsion of 

 the embryo takes place from the head region posteriorly, so that cross 

 sections made from the anterior end will show the embryo turned upon 

 its left side, while in the posterior region it still lies upon its ventral 

 surface. 



The flexion continues also, so that the mid-brain becomes the most 

 anterior region of the embryo. This flexion not only brings the fore- 

 brain in close relation to the heart, but brings optic and otic vesicles 

 opposite each other, it being remembered that the eye-pits form in the 

 fore-brain and the auditory pits in the medulla oblongata. 



THE NERVOUS SYSTEM 



All parts are growing, and have become larger than on the second 

 day (Fig. 288). The important new developments are as follows: the 

 epiphysis appears as a small evagination in the midline on the dorsal 

 surface of the diencephalon. It later becomes the pineal gland. 



Rathke's Pocket (Fig. 301, 1) is an ectodermal invagination which 

 has folded in just beneath the infundibulum. This pocket soon loses its 

 connection with the outer epithelium, and then becomes permanently 

 associated with the infundibulum to form the hypophysis or pituitary 

 body. 



THE OPTIC VESICLES (Fig. 289) 



It will be remembered that these were originally broad stalks 

 directly continuous with the cavity of the fore-brain. The cavity or 

 lumen of the optic stalks is then called an optocoele and the cavity or 

 lumen of the prosencephalon is called the prosocoele. A constriction 

 formed earlier is very marked at about fifty-five hours. The distal ends 

 have invaginated, forming a double-layered cup. The newly indented 

 layer is termed the sensory layer, because it is from this that the sensory 

 layer of the retina is to be formed. The underlying layer is called the 

 pigment layer, because it is from this that the pigmented layer of the 

 retina is to arise. The invaginated cups are often called secondary optic 

 vesicles to distinguish them from the original vesicles before invagina- 

 tion, the original vesicles being then known as primitive vesicles. 



The optic cup does not invaginate so as to form an equally rounded 

 edge. The invagination begins at the ventral surface and continues 

 dorsally and toward the midline, so that at the place where the invagi- 

 nation began, there is a region which has no definite lip. The cup, there- 

 fore, looks more like a cup that has had this portion broken out. This 

 lipless region is known as the choroid fissure. 



The invagination continues for the length of the optic stalk, thus 



