136 EMBRYOLOGY 



Figure 78 shows a cross section which passes through the ear and heart 

 and is schematic as regards the heart and blood vessels. The hindbrain has 

 very thick lateral walls, while the roof is very thin. The chief derivatives of 

 the hindbrain are the cerebellum, the medulla oblongata, and some of the 

 cranial nerves. On each side of the hindbrain a pear-shaped vesicle, the 

 otocyst, has developed from the epidermis. The otocyst begins as a thickening 

 of the epidermis (the auditory placode) which develops a pit, and the pit 

 finally becomes a closed cavity. The hollow otocyst separates from the 

 epidermis and floats in a mass of mesenchyme cells. A dorsal outgrowth of 

 the otocyst forms the endolymphatic duct. The walls of the otocyst differen- 

 tiate into the semicircular canals, which function as an organ of balance, 

 and also into the lagena, which is the organ of hearing. The auditory, or 

 eighth cranial, nerve originates from a mass of cells derived in part from 

 the neural crest and in part from the auditory placode. 



The pharynx is large in diameter at this level of the embryo. Just anterior 

 to this section the thyroid originates from the floor of the pharynx. Between 

 the pharynx and the ectoderm is a mass of mesenchyme through which the 

 aortic arches extend. Circulation begins about this time. In Figure 78 the 

 heart is shown diagrammatically, with the ventral aorta branching and giving 

 rise to the aortic arches, which extend dorsally around the pharynx and join 

 the roots of the dorsal aorta. Little blood flows through the aortic arch 

 directly, since a secondary loop develops through the external gill filaments 

 and most of the blood flows through these loops, where it takes up oxygen 

 and gives off carbon dioxide. The course of the circulation, beginning with 

 the heart, is as follows: through the afferent branchial artery to the vascular 

 loops in the gills, thence through the efferent branchial artery to the root of 

 the dorsal aorta. At this point part of the blood flows forward and is collected 

 and returned to the heart by the anterior cardinal veins. The remainder of 

 the blood flows posteriorly through the dorsal aorta into the developing 

 viscera. From the viscera the blood is returned to the heart through the 

 posterior cardinal vein. A more detailed treatment of the development of 

 the circulatory system will be found in Chapter 12. 



Figure 79 represents a section cut through the pronephros, which begins 

 to function at about the time of hatching. The pronephros arises as a series 

 of outgrowths from the nephrotome in the second, third, and fourth seg- 

 ments. One end of each outgrowth turns posteriorly and fuses with the next 

 to form a pronephric duct, which then grows backward to join with the 



