ORIGIN OF THE URO-GENITAL SYSTEM. 



805 



Wolffian duct, which terminates on each side in the cloaca. The 

 Maipighian glomeruli were first discovered by Kathke, who pointed out 



Fig. 599. — Human Embryo of from 25 to 28 days, viE\yED 



FROM BEFORE, THE ThORAX AND AbDOMEN OPENED (frOlll 



KoUiker after Coste). 



0, tlie eye ; in, the maxillary plate ; mn, the inferior 

 maxillary plate ; h, the second postoral plate ; h, the heart ; 

 w, Wolttiaii bodies and ducts on their outer borders ; I, the 

 liver ; c', the upper and t", the lower limbs ; a, the allantoid 

 pedicle, and on each side of it the umbilical arteries ; i, i, 

 tue upper and lower parts of the intestine of which the 

 middle parts with the vitello-intestinal duct have been removed, 

 leaving the mesentery sti'etched between. 



Fig. 599. 



their vascular structure, and their vessels derived 

 from neighbouring branches of the aorta. The 

 ducts of the Wolffian bodies are found to contain 

 a whitish fluid, and the bodies are believed to 

 perform the glandular office of kidneys during a 

 part of foetal life in the higher vertebrata, and 

 they have accordingly received the name of 

 irrimonlial Iciihieys, a designation which is quite 

 appropriate, as it appears that in fishes and am- 

 phibia, they remain as the whole in some, and a 

 part in others, of the permanent kidneys. 



In the human foetus they begin to be formed 

 along wuth the allantois, at a very early period, 

 probably before the third week, as they are already 

 very apparent in the fourth. They have attained 

 their full size by the sixth week, and in the 

 seventh and eighth are rapidly diminishing in 

 size, in connection with the changes which accompany the develop- 

 ment of the genital organs and the permanent kidneys. 



Fig. 600. 



Fig. 600. — Transverse Section through: the Embryo of the Chick and Blasto- 

 derm ON THE Second Day (from Kolliker). 

 d d, hypoblast ; ch, chorda dorsalis ; u w, primordial vertebrae ; m r, medullary 

 plates ; h, corneous layer or epihlast ; u lo h, cavity of the primordial vertebral mass ; 

 in p, mesoblast dividing at s p into h p I, somatopleure, and d f, splanchnopleui'e ; 

 un g, Wolffian duct, beginning in the intermediate cell-mass. 



As development advances the "Wolffian bodies rapidly become pro- 

 portionally shorter and thicker : they shrink towards the lower part of 

 the abdominal cavity, and soon become almost entirely wasted. By 



