THE RESPIRATORY AND URINARY ORGANS. 135 



from the Wolffian bodies, the stomach, and the liver, whilst the heart is still 

 near the head. As the diaphragm extends forward from the vertebral column it 

 separates the common pleuro-peritoneal cavity into two parts, a thoracic and 

 abdominal. 



Development of the Urinary Organs. Three distinct sets of urinary organs 

 occur in the embryo at different periods of development, two of them being more 

 or less transitory, while the third becomes the permanent kidney. The first to 

 appear is the pronephros or head-kidney and it consists of a small number of some- 

 what convoluted tubules which develop immediately behind the heart in the 

 mesoblast of certain of the protovertebral somites. The tubules are segmentally 

 arranged, one corresponding to each protovertebra, and they communicate at one 

 extremity with the ccelom and at the other with a longitudinal canal known as 

 the 9egmental or Wolffian duct. Later the second kidney appears below the 

 pronephros, developing in a similar manner and forming the mesonephros or 

 Wolffian //<///. whose tubules are also at first arranged segmentally, though later 

 they become more numerous than the protovertebrce from which they arise, by the 

 formation of secondary and tertiary tubules by budding from those already pres- 

 ent. These tubules likewise communicate with the Wolffian duct, and in connection 

 with each of them there is developed a little knot of blood-vessels which projects 

 into the lumen of the tubule, whose wall it pushes in front of it, and forms the 

 Malpigkian l>o<h/ or glomeruhis. The third and last kidney to appear is the meta- 

 nephros or permanent kidney, which, together with the ureter, arises as an out- 

 growth from the lower end of the Wolffian duct, 



The Wolffian duct is perceptible about the third week, forming an elongated 

 ridge of cells situated on either side of the primitive vertebra and extending from 

 the heart to the lower end of the embryo. It makes its appearance below the 

 heart and behind the common pleuro-peritoneal cavity, from the mesoblast at the 

 point of separation of its two layers into somatopleure and splanchnopleure, this 

 portion of the mesoblast being termed the "intermediate cell mass." The ridge 

 is at first solid, but soon a tube is hollowed out in it, and continuing to develop 

 posteriorly it unites with the proximal end of the allantois which forms what is 

 termed the urogenital sinus. Thus a communication is established through the 

 Wolffian tubes and ducts between the pleuro-peritoneal cavity and the cloaca or 

 hinder part of the alimentary canal. The next step is the formation of a second 

 duct in the neighborhood of the original duct, with which some of the tubules of 

 the anterior part of the segmental body (pronephros) are connected. This is the 

 Miillerian duct. The ureter, which is formed later, is, as has been described, an 

 offshoot from the hinder part of the Wolffian duct. 



The structure of the Wolffian body is in many respects analogous to that of 

 the permanent kidney (Fig. 104). It is composed partly of an excretory canal 



or duct, into which open numerous " con- 

 duits," rectilinear at first, but afterward 

 tortuous, and partly of a cellular or 

 glandular structure, in which Malpig- 

 hian tufts are found. It is fixed to the 

 diaphragm by a superior ligament, and 

 to the spinal column by an inferior or 

 inguinal ligament. Its office is the same 

 as that of the kidnes viz. to secrete 



FIG. 104. Enlarged view from the front of the flu if] pnntaininrr nrpa \vnipVi 

 left Wolffian body before the establishment of the "Ilia containing Uiea. WHICH 



distinction of sex. (From Farre, after Kobeit.) a, a, m the bladder. u hen the permanent 



b, d. Tubular structure of the Wolffian bodv. e. -, . . c i i c 



Wolffian duct. /. its upper extremity. <j. its termi- kidnevs are formed, the greater part ot 



nation in x, the urogenital sinus, h. The duct of -i \\* loc t J J'^ TV^ 



Muiier. /. its upper, stm closed, extremity. t. its the \\ olthan body disappears. Ine rest 



lower end, terminating in the urogenital sinus. /. tatps rnrt in thp formation nf thp nro-an 

 The mass of blastema for the reproductive organ, taKes P art m tne me OlganS 



ovary or testicle. of generation. 



The functional activity of the Wolf- 

 fian bodies is very transitory ; they attain their highest development by the sixth 



