338 ANATOMY OF THE DOMESTIC FOWL 



forms the future apex of the heart. If the series -of sections be 

 studied from the posterior forward, the ventricle will be first to 

 appear in the sections. Just beneath the auricular portion of the 

 heart there is the bulbus arteriosus. The bulbus arteriosus soon 

 divides into a number of aortic arches. There is an ascending pair in 

 each of the visceral arches. The dorsal aorta is formed by the union 

 of the aortic arches above the visceral arches. The aortic arches are 

 first continued a short distance forward as the carotid arteries. The 

 dorsal, or posterior, aorta passes backward under the notochord. 

 The dorsal aorta divides into two parallel aortae which give off on 

 each side the vitelline arteries. Note other branches of this aorta. 



The veins at this stage consist of the anterior and posterior cardi- 

 nal, the ductus venosus, the ductus Cuvieri, and the vitelline veins. 



Make drawing of the circulatory system after a completion of the 

 study of the series of sections. 



Make a study of the nervous system according to hints already 

 given. ^ 



The dorsal and the ventral roots of the spinal nerves are given off 

 separately, and secondarily unite. From the neuroblasts of the cord 

 there are at regular intervals outgrowths representing the ventral 

 roots. From the neural crest there develop segmental collections 

 of neuroblasts which form the spinal ganglia, from which the dorsal 

 spinal nerve roots develop. In fact this developmental stage can 

 be observed in embryos only forty-eight hours old, first appearing 

 as a line of ceUs springing on each side from the angle between 

 the neural canal tube and the external epiblast. In the section 

 from the embryo seventy-two hours old there are observed the 

 rudiments of the development of these spinal nerves. Four pri- 

 mary ganglia develop in the neural crest of the head. These 

 gangha are as follows: the acoustico-facialis ganglia, which is located 

 over the hyoid arch; the gangha of the trigeminus, which is located 

 over the mandibular arch; the ganglia of the glosso-phar3mgeus, 

 which is located over the third visceral arch; and the gangha of the 

 vagus, which is located over the third and the fourth visceral cleft. 



In this stage of development the trigeminal and the acoustico- 

 faciahs are clearly visible. 



On each side and dorsal to the aorta is noted the Wolffian body, 

 or mesonephros. The Wolffian body (Fig. 82 E) consists of a 

 series of tubules imbedded in the mesenchyme. The openings into 

 the Wolffian duct he just beneath the cardinal vein. 



