280 



ANIMAL PHYSIOLOGY. 



before birth. The germinal vesicle makes its appearance 

 first, then the rest of the ovum; the ovum subsequently 

 becomes imbedded in corpuscular matter, and this becomes 

 separated by imbibition of fluid into two strata, one of which 

 adheres to the ovum, and is the discus proligerus alluded to, 

 while the other adheres to the ovisac, and is named membrana 

 granulosa. In their early stages of development, the ova 

 move from the circumference towards the attachment of the 

 ovary, the smallest ova being found close to the peritoneum. 

 It is only when the Graafian vesicles begin to fill with 

 fluid that they push their way in the direction of least 

 pressure, precisely as an abscess 

 would, and thus approach the peri- 

 toneum again, at the same time that 

 the ovum quits the centre of the 

 vesicle, and adheres to the outer 

 wall. The originally centripetal 

 movement reminds us that although 

 in all the higher vertebrata, and 

 many fishes, the ova escape by peri- 

 toneal rupture, yet in the majority 

 of osseous fishes they grow in fes- 

 toons, directed to the centre of a 

 hollow organ, which opens by a duct, 

 like a secreting gland. Also the 

 homologous organ in the male is a 

 secreting gland, in which the secre- 

 tion travels from the circumference 

 towards the attachment; and a rudi- 

 mentary appearance of secreting 

 Fig. 143. OVARY AND tubules, in which the ova are de- 



(Ortliraoforiscus mola) . i y 



laid open. malian ovaries by some observers. 



Thus the ovary may be regarded as 

 an imperfectly developed secreting gland. 



At periodic intervals of a month's duration, one or more 

 Graafian vesicles ripen, rupture, and discharge their contents. 

 At the same time, the mucous membrane of the uterus shares 

 in the vascular excitement which the ovaries exhibit, and 

 this passes away with an extravasation of blood from thg sur- 



