THE ORGANS OF THE MIDDLE GERM-LAYER. 385 



NAGEL has studied the development of the testis in human embryos. Accord- 

 ing to his description also, there arise from the actively proliferating germinal 

 epithelium numerous cords, in which large primitive seminal cells are imbedded. 

 The cords afterwards become the seminal tubules. In Man there prevails from 

 the beginning, as NAGEL remarks, such a great difference between the two 

 sexes, both in the form of the original germinal ridge and in the whole process 

 of its differentiation, that one can recognise in the anatomical structure of the 

 sexual glands from a very early stage whether one has before him a male 

 or a female. 



(h) Metamorphosis of the Different Fundaments of the Urogenital 

 System into the Adult Condition. 



We have become acquainted in the preceding pages with the first 

 development of the various parts which constitute the foundations of 

 the urogenital system. These are (fig. 219) three pairs of canals 

 the mesonephric ducts (ug), the Miillerian ducts (mg), and the 

 ureters (hi) and in addition a great number of glandular structures 

 pronephros, mesonephros (un\ metanephros (ra), and the sexual 

 glands (kd), ovary and testis. 



It will be my task in what follows to indicate how the ultimate 

 condition is derived from these embryonic fundaments. In this I 

 shall limit myself, in the main, to Man, because we now have to do 

 with more easily investigated, and in general well-known conditions. 



In a human embryo eight weeks old (fig. 220) the fundaments,, 

 if we neglect differences which are recognisable only by the aid 

 of the microscope, are so similar in male and female as to be 

 indistinguishable. 



All the glands lie at the sides of the lumbar vertebrae : farthest 

 forward the kidney (n), which is a small bean-shaped body ; upon 

 this lies the suprarenal body (nn), that at this time is dispropor- 

 tionately large and is to be seen only on the left half of the figure. 



Somewhat lateral to the kidney one sees the primitive kidney (un) 

 as an elongated, narrow tract of tissue. It is attached to the wall 

 of the trunk by a connective-tissue lamella, a fold of the peritoneum, 

 the so-called mesentery of the primitive kidney. In the middle of 

 the gland it is rather broad, but above, toward the diaphragm, it 

 is elongated into a narrow band, which KOLLIKER has described as 

 the diaphragmatic ligament of the primitive kidney. Upon careful 

 examination one also observes at the lower end of the primitive 

 kidney a second fold of the peritoneum, which runs from it to the 

 inguinal region (figs. 219 and 220 gh). It encloses a firm strand 

 of connective tissue, a kind of ligament, that is destined to play a 



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