REPRODUCTION 



623 



V. Impregnation. 



Impregnation is effected by the transmission of spermatozoa 

 into the genital tract of the female. For this purpose erection 

 of the penis is brought about reflexly through a centre in the 



Fig. 237. — Ovum, after Segmentation, showing the Formation of the Ectoderm 

 (.4.) and Endoderm {B.). From the cells of the latter the Blastoderm 

 is formed. (Ellenberger.) 



lumbar enlargement of the cord, the outgoing nerves being the 

 nervi erigentes, or pelvic nerves which dilate the arterioles, and 

 the internal pudics supplying the transversus perinei and bulbo- 

 cavernous muscles by which the veins of the penis are constricted. 

 The semen is ejected by a rhythmic contraction of the bulbo- 



.-1. 



B. 



C. 



Fig. 238. — To show A., the Spreading out of the Endoderm Cells to Form the 

 Blastoderm ; B., the Formation of Epiblast and Hypoblast ; and C, of 

 Mesoblast. In B. and G. the ectoderm is not shown. (Ellenberger.) 



cavernous and other perineal muscles, an action which is also 

 presided over by a centre in the lumbar region of the cord 

 (p. 89). 



The spermatozoon meets the ovum in the Fallopian tube or 

 u pper part of the uterus. 



