Jones, Development of the Sympathetic. 1 15 



the researches of Onodi ('86), 011 elasmobranchs. In the human em- 

 bryo, His, Jr. finds that the development of the sympathetic begins (in 

 a 10 mm. embryo) with the outgrowth of the white rami communican- 

 tes from the spinal nerves. A little later, the sympathetic cord appears 

 and is joined by the rami. These findings contrast with those of Pat- 

 ER8ON ('91), who observed that the structure first to appear is the sym- 

 pathetic cord, the rami developing later. 



Hoffmann's observations on the sympathetic system of selachians 

 ('99) agree, as far as essential features of development are concerned, 

 with those of Balfour and Onodi. Like them, he shows that the 

 cells forming the sympathetic aulage arise in connection with the spi- 

 nal nerves, thus favoring the view that the sympathetic is epiblastic in 

 origin. In urodeles, he finds ('02) the first trace of the sympathetic 

 occurring as scattered cells connected by slender rami to the ventral 

 branches of the spinal nerves. He does not express an opinion as to 

 whether these cells are of epiblastic or mesoblastic origin. In 30 mm. 

 salamanders, the sympathetic has come to be a continuous chain, in 

 some places fibrous, in other places cellular in character, extending 

 from the first spinal nerve back to the tail region and connected by 

 rami to all of the spinal nerves. 



Another question relates to the connection, during embryonic 

 stages, between the sympathetic anlage and the adrenals. These two 

 structures come into very intimate relation with each other during their 

 development. In reptiles, there arises on each side of the vena cava 

 soon after its formation a longitudinal cord of cells, which Minot calls 

 the "mesenchymal anlage" of the adrenals. On the dorsal side of this 

 anlage and somewhnt toward the median line, appear clusters of cells, 

 which are derived from the sympathetic ganglia. They constitute the 

 "sympathetic anlage'" of the adrenals. 



These two portions come in contact, and at first, in amniota at 

 least, the sympathetic part grows more rapidly, and partially surrounds 

 the mesenchymal portion ; but soon the relations become reversed, and 

 the mesenchymal portion gradually surrounds the sympathetic ; what 

 finally becomes of the latter is not known. Balfour ('78) found in 

 elasmobranchs, in the case of the posterior adrenals, that each of these 

 bodies shows a small sympathetic ganglion attached to either end of it, 

 the whole structure being attached to a spinal nerve by a ramus. This 

 mass of cells gradually becomes divided into a ganglionic and a glandu- 

 lar portion ; the latter acquiring a mesoblastic investment becomes 

 adrenal, while the former develops into sympathetic tissue. Hoff- 



^" Human Embryology." Pages 48 j and 4.86. 



