CHAPTER XXXI 

 FERTILIZATION AND KARYOKINESIS OF THE OVUM 



Fecundation Maturation of the ovum Fertilization of the ovum Mendel's laws of hered- 

 ity Superfecundation Segmentation of the ovum Gastrulation Primitive streak 



Formation of the membranes Formation of the amnion Amniotic liquid Forma- 

 tion of the umbilical vesicle (yolk-sac) Formation of the allantois and permanent chorion 



Membranae deciduse Formation of the placenta Uses of the placenta. 



So far as the male is concerned, coitus is rendered possible by erec- 

 tion of the penis. This may occur before puberty, but at this time 

 intercourse can not be fruitful. Coitus may be impossible in old age, 

 from absence of the power of erection ; but spermatozoids may still 

 exist in the vesiculae seminales, and fecundation might occur if the sem- 

 inal fluid could be discharged into the generative passages of the female. 

 Coitus may take place in the female before the age of puberty or after 

 the final cessation of the menses, but intercourse can not then be fruit- 

 ful. There are many instances of conception following what would be 

 called imperfect intercourse, as in cases of unruptured hymen, deformi- 

 ties of the male organs, etc., which show that the actual penetration of 

 the male organ is not essential, and that fecundation may occur provided 

 the seminal liquid find its way into even the lower part of the vagina. 

 Conception also has followed intercourse when the female has been in- 

 sensible or entirely passive. Unlike certain of the lower animals, the 

 human subject presents no distinct periodicity in the development of 

 the spermatozoids ; but in reiterated connection, an orgasm may occur 

 when the ejaculated liquid has no fecundating power. 



In regard to the mechanism of erection, little remains to be said 

 after the description that has been given of true erectile tissue in con- 

 nection with the physiology of the circulation. The cavernous and 

 spongy bodies of the penis usually are taken as. the type of erectile 

 organs. In these parts -the arteries are large, contorted, provided with 

 unusually thick muscular coats and are connected with the veins by 

 vessels considerably larger than the true capillaries. They are sup- 

 ported by a strong fibrous network of trabeculae, which contains non- 

 striated muscular fibres ; so that when the bloodvessels are completely 

 filled the organ becomes enlarged and rigid. Researches in regard to 

 the nerves of erection show that the vessels of erectile tissues are dis- 



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