194 EMBRYOLOGICAL TYPES 



There are two kinds of placodes : an upper row of dorso- 

 lateral placodes which give rise to the lateral-line sense- 

 organs and to the nerve-cells whose fibres innervate them ; 

 a lower row of epibranchial placodes situated at the dorsal 

 ends of the visceral slits, and which give rise to the nerve- 

 cells whose fibres innervate the sense-organs of taste. 



Sympathetic System and Adrenals. — The dorsal nerve- 

 root, formed by fibres which have grown out from cells in the 

 dorsal-root ganglion, and the ventral nerve-root which has 

 grown out from the spinal cord, join to form a mixed nerve. 

 Certain cells migrate out from the spinal cord, and, leaving the 

 mixed nerve, make for the side of the dorsal aorta where they 

 form the sympathetic ganglia. These ganglia remain con- 

 nected with the mixed nerve by the rami communicantes. The 

 sympathetic ganglia are, like the mixed spinal nerves, seg- 

 mentary arranged. They soon become connected by fibres 

 running to the (sympathetic) ganglia in front and behind them 

 forming the sympathetic trunks. From the sympathetic 

 ganglia, " postganglionic " fibres are distributed to the smooth 

 muscles of the gut, oviducts, and blood-vessels. Other cells 

 migrate out from the sympathetic ganglia, and give rise to the 

 medulla of the adrenal bodies. The cortex of these bodies 

 is derived from the ccelomic epithelium in the region between 

 the mesonephric kidneys. 



It may be mentioned that cells migrate out from the hind- 

 brain along the vagus and eventually come to lie on the surface 

 of the heart and gut, forming part of the parasympathetic 

 system. 



Skeleton. — The vertebral column arises in the form of 

 paired cartilages beside the notochord, derived from the 

 sclerotomes. Each vertebra arises opposite the septum 

 separating two segments ; the vertebrae are therefore inter- 

 segmental in position. 



In the skull, paired trabecular arise as struts underlying the 

 forebrain, and, behind them, paired parachordals flank the 

 notochord. The ptery go- quadrate or skeleton of the upper 

 jaw arises early, and fuses on to the remainder of the skull by 

 its ascending process. The auditory sac becomes surrounded 



