ri;<>< JKMTAL SYSTK.M 177 



form the centro-acinar cells (Laguesse). Th cell-islands of Lan^erhans have been gener- 

 ally iviranlo I as derivatives of the mesenchyme. This view has been a.l\ o. atcd by Hansemann 

 ( I!oi2), hut it has been shown by Laguesse, Kiister, Pearce, and Helly that they are derived from 

 tin- gland-epithelium. 1 Helly has traced (in the guinea-pig) the Langerhans cells back to a 

 sta^o in which the pancreas-rudiment is still a solid bud. These special cells are characterised 

 by dense finely granular protoplasm; they do not share in the formation of the tubules, but 

 lie loosely arranged in their walls. In the human embryo (Pearce) such cells occur in round or 

 oval masses and in direct continuity with the epithelial cells. They bud off and form solid 

 cell- processes which are at first connected with the acini, but later become separated from them 

 by the ingrowth of mesenchyme. The group thus isolated is vascularised, and by further 

 histogenetic changes becomes a fully formed cell-islet. 



DEVELOPMENT OF THE UEOGENITAL SYSTEM. 2 



EXCRETORY ORGANS. 



The excretory system first appears as a longitudinal duct and a longitudinal 

 series of epithelial tubules in that part of the mesoderm named the intermediate 

 cell-mass. This forms a continuous blastema from which every part of the 

 system takes origin. It consists at first of cells disposed, though somewhat 

 indistinctly, in two lamellae outer and inner which are connected with the 

 outer and inner layers of the primitive segment internally, and the somatopleuric 

 and splanchnopleuric layers of the lateral plate externally. 



In the Anamnia the corresponding portion of the mesoderm forms in each segment a hollow 

 stalk derived from the ventral part of the segment, through which the myocoel is 

 continuous with the splanchnocoel (general body-cavity). In the mammalia, owing to the 

 condensation of development, the stalks are solid like the segments themselves, and all traces of 

 segmentation are lost at a very early stage. 



In the nephrogenetic blastema the tubules are developed from before back- 

 wards, and are grouped, as they appear in time and place, into three systems, the 

 pronephros, mesoncphros, and metanephros. 



Pronephros and seg mental ( Woltfian) duct. The pronephros is developed 

 as a functional organ in the Anamnia during the larval stage. It is rudi- 

 mentary in the Amniota, and represented only by vestiges in the Mammalia. When 

 typically developed it consists of a number of coiled tubules, which, joining one 

 another at their outer ends, form the segmental duct, while at their inner ends 

 they open into a chamber, the mesial wall of which is invaginated by a glomerulus 

 i.e. a tuft of capillary vessels derived from a branch of the aorta. The chamber 

 is formed (Brauer in Gymnophiona), by a folding of the mesoderm-layers, from 

 the ventral part of the hollow primitive segment. It becomes cut off from the 

 myocoel, but remains connected by a passage with the splanchnocoel (body- 

 cavity). The tubule is formed as a diverticulum of the chamber by a folding 

 of the somatic layer of mesoderm, while the peritoneal passage becomes the 

 nephrostome. 



In the human embryo a very rudimentary pronephros is probably to be 

 recognised in a longitudinal duct, some blind tubules, a peritoneal funnel, and 

 a vestigial glomerulus, which have been described by several observers, 3 in the fifth, 

 sixth, and seventh segments. It is possible, however, that these vestiges may 

 represent degenerating mesonephric structures. 



1 Hansemann, Verh. deutsch. pathol. Ges. 1901 ; Laguesse, loc. cit. (Hertwig), and Arch. d'Anat. 

 microsc. v. 1902 ; Pearce, Amer. Jour, of Anat. ii. 1903 ; Ktister, Arch. f. mikr. Anat. Ixiv. 1904 ; Helly, 

 ibid. Ixvii. 1905. 



2 For literature up to 1905, see Felix, Hertwig, Entwickelungslehre, III. Teil i. and ii. p. 852 seq. 



" Janosik, Arch. f. mikr. Anat. xxx. ; Tandler, Anat. Hefte, xxviii. ; MacCallum, Amer. Jour, of 

 Anat. i. ; Gage, ibid. iv. ; Keibel, Anat. Anzeiger (Ergiinzungsheft), xxvii. 1905 ; Ingalls, Arch. f. mikr. 

 Anat. Ixx. 1907. 



VOL. I. N 



