S4 K. MITSUKURI. 



Other mesoblastic cells, but ventrally its termination is in- 

 definite. This mass is probably the first rudiment of the 

 mesoblastic or cortical part of a suprarenal body. 



On the fourteenth day the suprarenals are already well 

 marked. Fig. 4 shows the left suprarenal (s, r.) a few 

 sections behind the front end. It consists of a mass of cell 

 with large nuclei, divided into indefinite cords by blood 

 capillaries which are already somewhat numerous. Cell 

 limits are hardly visible. Mesoblastic cells surround the 

 mass as a capsule. Dorsal to the suprarenal is placed 

 another mass of cells (n) looking very much like the former, 

 but if anything, with slightly smaller nuclei stained darker. 

 This, on tracing it forward, is found continuous with the sym- 

 pathetic cells {sytnp., fig. 4), although at this stage the con- 

 nection is not so obvious as later on. 



The subsequent history shows that the medullary sub- 

 stance arises out of the above ventral mass (?^) of the sym- 

 pathetic system. From following the sections the suprarenal 

 is found to be widest in the middle, and to taper both in 

 front and behind. The cords of which it is composed are 

 tolerably distinct, and the blood channels between them pro- 

 portionally very wide and conspicuous. The sympathetic 

 mass {n) gradually extends ventrally between the aorta and 

 the suprarenal, and is continued back far beyond the poste- 

 rior end of the suprarenal. It spreads, in some sections, 

 around the latter, and in such cases it is difficult to tell the 

 two bodies apart, as their structure at this early stage is 

 very similar. It is only by tracing them back that they can 

 be distinguished. The suprarenal and the sympathetic 

 masses of the two sides remain, however, separate 

 throughout. 



In the suprarenals of sixteen-day embryos, great 

 changes are observable. The medullary part (m, fig. 5) 

 surrounded for the most part by the cortical substance (c) 

 can now be clearly distinguished. In the medulla the nuclei 

 of the cells are stained darker than in the surrounding 

 ]5art, and the medulla itself is at this stage nothing but a 

 mass of simple cells with a mixture of a great number of 

 spindle cells and other connective-tissue elements. The corti- 

 cal substance is still made up of irregular cords of cells without 

 any more definite structure. Closely applied against the supra- 

 renal, on its inner side, there is a large mass of sympathetic 

 nerve cells (??). At its ventral end, a process of this mass, 

 partly composed of nerve fibres, enters the cortical substance 

 at the point (a). The nervous mass has very much the appear- 

 ance of the medullary substance— its nuclei being stained in 



