32 HUMAN EMBKYOLOGY. 



life both apertures are closed and, for a time, the neural canal becomes a completely 

 closed cavity. 



As the margins of the neural groove rise and converge they carry with them 

 the adjacent ectoderm to which they are attached, and which forms part of the 

 surface covering of the embryo ; consequently, when the lateral margins of the 

 folds meet and unite, the tube, which is completed by their fusion, is embedded in 

 the body of the embryo, but, for a time, its dorsal wall is attached to the surface 

 ectoderm by a ridge of cells, formed by the fused lateral margins of the neural 

 plate. This ridge is called the neural crest (Figs. 41-44). 



The neural crest is the rudiment of the cerebral and spinal nerve ganglia, the 

 sympathetic ganglia, the chromaffin cells of the chromaffin organs, and the cellular 

 sheaths of the peripheral nerves; whilst the walls of the neural tube become 

 transformed into the various constituent parts of the central nervous system, the 

 brain and spinal medulla, the retinae of the eye-balls, and the optic nerves. 1 



The Formation of the Nerve Ganglia, the Chromaffin Tissues, and the 

 Primitive Nerve Sheaths. The primitive ganglia grow as cell buds from the 

 neural crest which, for a time, connects the dorsal wall of the neural tube with 

 the surface ectoderm. In the body region they correspond in number with the 

 spinal nerves and with the primitive segments into which the* mesoderm becomes 

 divided, but in the cephalic region their arrangement is more irregular, and some 

 of the ganglia of the cerebral nerves receive additional cell elements from the 

 surface ectoderm. 



Simultaneously with the appearance of the cell buds which form the primitive 

 ganglia, the neural crest disappears, and directly after the ganglia are formed they 

 lose their connexion with both the neural tube and the surface ectoderm and 

 become isolated cell clumps. At this period, therefore, the nervous system 

 consists of the neural tube and the primitive ganglia. 



After the primitive ganglia have lost their connexion with the neural tube 

 they increase in size by the proliferation of their constituent cells, and they 

 migrate ventrally along the sides of the neural tube, but the migration ceases 

 before the ventral ends of the ganglia reach the level of the ventral wall of 

 the tube. As the migration proceeds clumps of cells are budded off from the 

 ventral ends of the ganglia. These secondary cell buds are the rudiments of 

 the sympathetic ganglion cells and of the chromaffin tissue which is found in 

 the sympathetic nerve plexuses, the medulla of the suprarenal glands, and in the 

 carotid glands. In the first instance the secondary cell buds which form the 

 sympathetic ganglia wander ventrally and medially, from the ventral ends of 

 the primitive ganglia, until they attain the positions afterwards occupied by the 

 ganglia of the sympathetic trunks on the ventro-lateral aspects of the vertebral 

 column. From the primary sympathetic ganglia, buds of cells are given off; these 

 buds wander still further ventrally to become the cells of the ganglia of the cardiac, 

 coeliac, and other great ganglionic nerve plexuses, as well as to form the 

 chromaffin cells of the chromaffin organs. 



The exact manner in which the cells of the primitive sheaths of the nerves 

 originate from the primitive ganglia is not known, but it has been shown by 

 Harrison, in the case of the frog, that if the primitive ganglia are destroyed, 

 the primitive sheaths of the nerves are not formed. Presumably, therefore, in the 

 frog the cellular sheaths of the nerves are derived from cells produced by the 

 primitive ganglia, and it may be assumed that they have a similar origin in 

 the human subject. 



After the rudiments of the sympathetic system, the chromaffin cells, and the 

 cellular sheaths of the nerves have separated, the remains of the primitive ganglia 

 become the permanent spinal and cerebral nerve ganglia. 



In the early stages these ganglia are completely isolated structures which lie 

 along the sides of the neural tube between the lateral walls of the tube medially, 

 and the mesoderm somites laterally. 



Some time after the ganglia of the cerebral and spinal nerves become isolated 



1 It is stated that some of the sympathetic nerve-cells are derived from the ventral parts of the lateral 

 walls of the neural tube, but the evidence on this point is not entirely satisfactory. 



