REPRODUCTION 415 



the uterus break down, and are shed during the menstrual 

 period. After the menstrual period, the membrane again 

 regenerates. 



Impregnation is effected by the transmission of sperma- 

 tozoa into the passages of the female. For this purpose 

 erection of the penis is brought about reflexly through a 

 centre in the lumbar enlargement of the cord, the outgoing 

 nerves being the nervi erigentes, which dilate the arterioles, 

 and the internal pudics supplying the transversus perinei 

 and bulbo-cavernous muscles by which the veins are 

 constricted. 



The semen is ejected by a rhythmic contraction of the 

 bulbo-cavernous and other perineal muscles, an action which 

 is also presided over by a centre in the lumbar region of the 

 cord (p. 150). 



The spermatozoon meets the ovum in the Fallopian tube, 

 or upper part of the uterus. 



DEVELOPMENT. 

 1. Early Stage. 



It is unnecessary here to describe the changes in the 

 ovum before or immediately after its conjugation with 

 the spermatozoon, since 

 they are so fully dealt 

 with in all works on 



The mammalian ovum 

 is holoblastic, that is 

 undergoes complete seg- 

 mentation, and forms a 



mulberry -like mass of JSK^g B. 



cells (Fig. 157, A). The 



Cells then get disposed FIG. 157.^1. Ovum with central cells forming 



in tWO SetS, a layer of Blastoderm. B. Blastoderm now made up 



,. ,, of two layers of cells Epiblast and 



small surrounding cells Hypobiast. 



and a set of large central 



cells. These latter spread out at one pole to form the 



