IMPREGNATION. 841 



CHAPTER III. 



IMPREGNATION. 



781. IN coitus the discharge of the semen containing the spermatozoa 

 is most probably effected by means of the peristaltic contractions of the vesi- 

 cu\2d semiiiales and vasa deferentia, assisted by rhythmical contractions of 

 the bulbo-cavernosus muscle, the whole being a reflex act, the centre of 

 which appears to be in the lumbar spinal cord. In the dog emissions of 

 semen can be brought about by stimulation of the glans penis after complete 

 division of the spinal cord in the dorsal region. The emission of semen is 

 preceded by an erection of the penis. This we have already seen (p. 274) is 

 in part at least due to an increased vascular supply brought about by means 

 of the nervi erigentes ; it is probable, however, that the condition is further 

 secured by a compression of the efferent veins of the corpora cavernosa by 

 means of smooth muscular fibres present in those bodies. The semen being 

 received into the female organs, which are at the time in a state of turges- 

 cence resembling the erection of the penis, but less marked, the spermatozoa 

 find their way into the Fallopian tubes, and here (probably in its upper 

 *part) come into contact with the ovum. In the case of some animals im- 

 pregnation may take place at the ovary itself. The passage of the sperma- 

 tozoa is most probably effected mainly by their own vibratile activity ; but 

 in some animals a retrograde peristaltic movement travelling from the uterus 

 along the Fallopian tubes has been observed ; this might assist in bringing 

 the semen to the ovum, but inasmuch as these movements are probably 

 parts of the act of coitus, and impregnaton may be deferred till some time 

 after that event, no great stress can be laid upon them. 



As the result of the action of the spermatozoa on the ovum, the latter, 

 instead of dying as when impregnation fails, awakes to great nutritive 

 activity accompanied by remarkable morphological changes ; it enlarges 

 and develops into an embryo. 



782. [Preceding the time of the occurrence of the entrance of the 

 spermatozoon into the egg, certain histological changes have been observed 

 to occur, and in order thoroughly to understand these, as well as the changes 

 which follow in the ovum, it will first be necessary to review the histology 

 of the egg. 



The ovule is a minute cell, the wall being formed by a structureless, 

 transparent membrane, called the zona pellucida, or vitelline membrane. 

 Within this is the yolk, or vitellus, which consists of a granular semi-fluid 

 mass, having suspended in it a nucleus or germinal vesicle which contains a 

 nucleolus or germinal spot The germinal vesicle consists of a very delicate, 

 transparent, homogeneous membrane which encloses a fluid with granules, 

 suspended in which is an eccentric nucleolus of a granular and fibrillated 

 structure. 



783. Previous to the occurrence of the impregnation of the ovule a 

 very interesting series of changes have been observed to take place. Accord- 

 ing to Balfour, the first interesting point to be noticed is the migration of 

 the germinal vesicle toward the cell wall. The vesicular wall then becomes 

 wavy and gradually disappears, while at the same time the nucleolus or 

 germinal spot has undergone metamorphosis, so that what remains of these 

 structures is a spindle-shaped mass. One extremity of this mass gradually 

 projects through the cell wall and is thrown off as a polar vesicle. From the 

 other remaining portion a second polar vesicle is formed, the part of the 



