434 VETERINARY PHYSIOLOGY 



cycle may be divided into three periods: (1) The anoestrous 

 stage, during which the genital organs are at rest, the uterus 

 and Fallopian tabes small, arid the formation of Graafian 

 follicles in the ovary quiescent ; (2) the procestrous stage, during 

 which there is a rapid ripening of one or more Graafian follicles 

 and a congestion and swelling of the Fallopian tubes and uterus, 

 with sometimes haemorrhages into the uterine mucous mem- 

 brane ; (3) the cestrous stage, during which coition is performed, 

 and during which the Graafian follicle ruptures and sheds the 

 ovum (ovulation). This in some animals e.g. ferrets occurs 

 as the result of coition ; in others horse, cow, etc. indepen- 

 dently of it. Sometimes the ovum is not shed, and then ft and 

 the rest of the Graafian follicle atrophy. 



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 retiexly 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 of the penis 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. 160). 



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 spermato- 

 zoon, since they are so fully dealt with in all works on biology. 



The mammalian ovum is holoblastic, that is, undergoes com- 

 plete segmentation, and forms a mulberry-like mass of cells 

 (fig. 163, A.). The cells then get disposed in two sets, a 

 layer of small surrounding cells and a set of large central cells 

 (fig. 169, B.). The former constitute the Ectoderm and take 

 part in forming the processes or primitive villi by which the 

 ovum becomes attached to the maternal mucous membrane. 



