206 SUPRARENAL BODIES 



from the rudiment of the spinal ganglion, or perhaps from the medullary plate, along 

 with the ventral nerve-roots. The cellular groups in these earlier phases are not 

 ganglia, but masses of indifferent or mother- cells which differentiate in two directions, 

 some becoming ganglion cells, others chromophil or chromaffin cells (i.e. cells having 

 special affinity for salts of chromic acid and staining yellow therewith). Masses of 

 these indifferent cells invade the substance of the adrenal, undergo differentiation 

 into chromaffin elements, and collect in the central part of the body to form the 

 cell-groups of the medulla. The medulla begins to be marked off from the cortex 

 in the fourth monfh. The adrenal is rounded in early stages and larger than the 

 kidney (fig. 170). In the third month it becomes flattened and triangular in section 

 (fig. 222), but remains relatively large all through foetal life. 



The development of the adrenals has been the subject of much discussion. Opinion has 

 been divided both as to the cortical and medullary parts of the glands. All have agreed 

 that the cortex arises from the mesoderm, but there have been three main views as to the exact 

 origin of the cells, some observers tracing them to the general mesenchyme, others to the 

 epithelium of the excretory ducts, and others to the mesothelium. The latest work of Wiesel 

 and of Soulie, confirming that of Janosik and Inaba, affords very strong support to the view 

 that cells are budded off from 'the peritoneal epithelium. ! Recent work, more especially that of 

 Kohn on the chromaffin elements, has thrown much light on the old problem of the medulla, 

 and it may be now taken as proved that Balfour's view of the origin of the medulla from the 

 sympathetic is the correct one. The close association with the sympathetic accounts also for the 

 presence of ganglion cells under the capsule, at the hilum, and in the medulla of the organ. 



DEVELOPMENT OF THE VASCULAK SYSTEM. 

 THE HEART. 



In an earlier section the initial phases in the development of the vascular 

 system have been described. We there saw that the heart was first laid down as 

 two tubes which unite together to form a single mesial tube. This lies under the 

 fore-gut in the pericardium, united to its dorsal wall by a double fold named the 

 dorsal mesocardium. The mesial tube is divided by constrictions into three 

 portions the primitive auricle, ventricle, and aortic bulb. Behind it is connected 

 with the sinus venosus, a transverse commissural vessel in the septum transversum 

 which receives three pairs of veins the vitelline, allantoic, and ducts of Cuvier. 

 In front it is continued into a ventral stem the truncus arteriosus which divides 

 into two vessels. These, looping round the fore-gut, are continued backwards on 

 each side of the middle line as the primitive aortse. 



The simple heart-tube soon becomes bent on itself. This bending is due to its 

 increasing in length disproportionately to the pericardial cavity which encloses it. 

 The dorsal mesocardium disappears between the primitive auricle and the attach- 

 ment of the bulb to the floor of the fore-gut. These two parts remain in close 

 relationship during all succeeding phases, but the free loop, becoming enlarged 

 and displaced backwards, comes to lie behind the original posterior end of the 

 tube. 



DEVELOPMENT OF THE OUTWAKD FOEM OF THE HEART. 



In the following account we shall start with the stage reached in the human 

 embryo by the fifteenth day (fig. 259) . The auricular portion of the heart-tube lies, 

 still attached by the mesocardium, immediately in front of the septum transversum. 

 From this point it is directed forwards and to the left. Reaching the anterior limit 

 of the pericardium, it turns ventrally to join, by a constricted portion named the 

 auricular canal, the ventral U-shaped ventricular loop. This consists (1) of a 



1 The literature of the development of the adrenals is very completely given in the article by Poll 

 in Hertwig's Handbuch der Entwickelungslehre, p. 603 seq. 



