DEVELOPMENT OF FOURTH DAY 519 



THE REPRODUCTIVE SYSTEM (Fig. 307) 



On the fourth day a thickened strip of peritoneum forms on the 

 lateral and superior face of the Wolffian body, which later extends all 

 the way to the cloaca. This may be called the tubal ridge. It appears 

 first at the anterior end of the Wolffian body and grows posteriorly, im- 

 mediately external to the Wolfnan duct. This tubal ridge invaginates, 

 forming a groove-like arrangement at the cephalic end of the Wolffian 

 body, and the lips of this groove then fuse and form a tube the 

 Mullerian duct. Fusion takes place on the fifth day. The anterior end 

 of this Mullerian duct remains open and is to become the opening into 

 the coelom of the oviduct which the Mullerian duct later becomes. There 

 are several openings which will develop at the anterior end in addition 

 to the main one, but these latter close normally. Should they remain 

 open, the abnormal condition of having two openings in the duct results 

 in the adult stage. The posterior end remains closed. 



The older embryologists considered these two or three openings in 

 the Mullerian duct as homologous with the nephrostomes of the 

 pronephros, and so insisted that the pronephros followed the 

 mesonephros in the chick. Modern embryologists consider that these 

 openings lie entirely too far posteriorly and laterally to permit of this 

 older interpretation. 



In both sexes so far, development has been alike, but on the eighth 

 day the Mullerian ducts begin to degenerate. They disappear almost 

 entirely by the eleventh day in the male. 



In the female chick, the left Mullerian duct forms the oviduct, while 

 the right Mullerian duct degenerates. The left one alone remains 

 functional. 



The Wolffian body disappears almost entirely in the male, though 

 a small group of tubules covering the anterior head of the testes remains 

 as the epididymis. In the female it also disappears almost entirely, the 

 part remaining being the parovarium, a small body lying in the mes- 

 entery between the ovary and the kidney. 



The Wolffian duct disappears entirely in the female, but it acts as 

 the vas deferens or sperm duct in the male. 



The germ cells probably arise from the entoderm in vertebrates. 

 The entoderm is never metameric, though some of the older embryolo- 

 gists spoke of metameric gonotomes as primitive segmented regions 

 which were to form the gonads. 



At about the time the somites form, the portion of the entoderm 

 which is to become the gonads, migrates through the developing meso- 

 derm in the epithelium of the genital ridges which have formed imme- 

 diately lateral to the mesentery. The primitive or primordial ova or 

 sperm can be recognized not only from their size but from their reactions 

 to microscopic stains (Fig. 254). 



