. The development of the chick; an introduction to embryology . These cells are at the distal entls of branchesof the vagus, with which they have grown into the heart. Pre-vious to this time these neuroblasts are found nearer to the vagusalong the course of the arteries. There can be but little doubtthat they have arisen from the vagus ganglion and that theyreach the heart by migration. Such an oi-igin has been demon-strated with great probability for all the known nervous elementsof the heart of the chick. (See Wilhelm His, Jr., Die Entwickelungdes Herznervensvstems bei Wirixlthioren.) 260 TH
. The development of the chick; an introduction to embryology . These cells are at the distal entls of branchesof the vagus, with which they have grown into the heart. Pre-vious to this time these neuroblasts are found nearer to the vagusalong the course of the arteries. There can be but little doubtthat they have arisen from the vagus ganglion and that theyreach the heart by migration. Such an oi-igin has been demon-strated with great probability for all the known nervous elementsof the heart of the chick. (See Wilhelm His, Jr., Die Entwickelungdes Herznervensvstems bei Wirixlthioren.) 260 THE DEVELOPMENT OF THE CHICK. ,■ o c CO *< < ^ -^ THE NERVOUS SYSTEM 261 If any cardiac nervous elements arise in situ, the} certainlyremain undifferentiated until those that have a ganglionic originhave already entered the heart. The Cranial Nerves. The nerves of the head exhiljit a muchgreater degree of heteronomy than the spinal nerves, and, inspite of much study, knowledge of their embryonic developmentis still in a very unsatisfactory condition. The same principles,however, apply to the development of both cranial and spinalnerves; the axones of the former like those of the latter ariseeither from medullary or ganglionic neuroblasts which are re-spectively unipolar and bipolar; but the cranial ganglionic and
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