THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



399 



found in or near the capsule of the gland. Others may consist of both cortical 

 and medullary substances, and are found in the vicinity of or embedded in the 

 kidneys, in the retroperitoneal tissue near the kidneys, in the walls of neighbor- 

 ing blood vessels, or associated with the internal genital 

 organs in the rete testis or epididymis, or in the broad 

 ligament. These accessory structures may arise inde- 

 pendently of the main gland, or they may be portions of 

 the main gland which were separated during the union 

 of the different anlagen of the latter and were carried 

 away in the descent of the genital glands. 



In addition to the chromamn tissue which enters into 

 the formation of the main gland or of accessory glands, 

 there are other small masses of this tissue which remain 

 permanently associated with some of the prevertebral 

 and peripheral sympathetic ganglia. 



Recent researches have shown that the Carotid Skein 

 (glomus caroticum, intercarotid ganglion, carotid gland), 

 which formerly was believed to be a derivative of the 

 epithelial lining of one of the branchial grooves, is of 

 sympathetic origin and that the cells acquire the charac- 

 teristic chromaffin reaction. These facts indicate that 

 it is closely allied with the medullary substance of the suprarenal gland. 



FIG. 352. Diagram of 

 the developing phaeo- 

 chrome . masses in a 

 human foetus of 50* 

 mm. A, Aorta; N ; . 

 cortical substance (in- 

 terrenal gland) ; U, 

 ureter; R, rectum. 

 Kohn. 



Anomalies. 



THE KIDNEYS. Rarely is there congenital absence of both kidneys. More 

 often there is a high degree of aplasia in both organs in otherwise well-developed 

 children. In either case death necessarily soon follows. Not infrequently one 

 kidney, usually the left, is poorly developed or absent and a compensatory 

 enlargement of the other exists. Such malformations are due to deficient 

 development of the organs, but the causes underlying the deficient development 

 are obscure. 



One of the most common malformations is the abnormal position of one or 

 both kidneys (ectopia of the kidneys). Usually they occupy a position lower 

 than the normal in the abdominal cavity, which indicates that they have failed, 

 during development, to migrate forward to the normal limit (see p. 369). Very 

 rarely one or both organs migrate beyond the normal limit, in which case they 

 occupy positions cranial to the normal. 



Not infrequently the lower ends of the two kidneys are fused across the 

 medial line, giving rise to the so-called "horseshoe kidney." Two renal 

 pelves and ureters are usually present. Occasionally the fusion is so extensive 



