FECUNDATION. 793 



homogeneous and scanty, and it may consist of a small quantity of secretion 

 from the vesiculae seminales and the glands opening into the urethra, with- 

 out spermatozoids and consequently deprived of fecundating properties. 



In old men the seminal vesicles may not contain spermatozoids ; but this 

 is not always the case, even in very advanced life. Instances are constantly 

 occurring of men who have children in their old age, in which the paternity 

 of the offspring can hardly be doubted. Duplay, in 1852, examined the 

 semen of a number of old men, and found, in about half the number, sper- 

 matozoids, normal in appearance and quantity, though in some the vesicular 

 seminales contained either none or very few. Some of the persons in whom 

 the spermatozoids were normal were between seventy-three and eighty-two 

 years of age. These observations were confirmed by Dieu, who found sper- 

 matozoids in a man eighty-six years of age. The contents of the seminal 

 vesicles, in these cases, were examined twenty-four hours after death. Some 

 of the subjects died of acute, and others, of chronic diseases ; but the mode 

 of death did not present any differences in the cases classed with reference 

 to the presence of spermatozoids. As the result of his own and of other 

 recorded observations, Dieu concluded that the power of fecundation often 

 persists for a considerable time after copulation has become impossible on 

 account simply of absence of the power of erection. 



CHAPTER XXV. 



FECUNDATION AND DEVELOPMENT OF THE OVUM. 



General considerations Fecundation Changes in the fecundated ovum Segmentation of the vitellus 

 Primitive trace Blastodermic layers Formation of the membranes Amniotic fluid Umbilical vesicle 

 Formation of the allantois and the permanent chorion Umbilical cord Membranes decidua? Forma- 

 tion of the placenta Uses of the placenta Development of the ovum Development of the cavities and 

 layers of the trunk in the chick Vertebral column Development of the skeleton Development of the 

 muscles Development of the skin Development of the nervous system Development of the organs 

 of special sense Development of the digestive apparatus Development of the respiratory apparatus- 

 Development of the face Development of the teeth Development of the genito-urinary apparatus 

 Development of the circulatory apparatus Description of the foetal circulation. 



As far as the male is concerned, coitus is rendered possible by erection 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 seminal 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 inter- 

 course can not then be fruitful. There are many instances of conception 

 following what would be called imperfect intercourse, as in cases of unrupt- 

 ured hymen, deformities of the male organs, etc., which show that the actual 



