BEPEODUCTION AND DEVELOPMENT. 



283 



which the future animal is developed. These being the facts, 

 the student may see that impregnation may be regarded as 

 the fusion of two mutually attracted units of life into one; 

 and that it is possible to consider the ancestry of every 

 nucleated corpuscle of the body as an unbroken chain, 

 through generations, from parents to children. 



Fig. 146. CLEAVAGE OF THE YELK. The dog, Bischoff. 

 In its passage downwards through the Fallopian tube, the 

 ovum undergoes some enlargement, and the zona pellucida 

 receives a coating of albuminous substance, gradually increas- 

 ing in thickness. On reaching the uterus, the envelope of 

 the ovum, henceforward called the chorion, proceeds to throw 

 out branching processes or villi, by which it becomes closely 

 connnected with the uterine walls, 

 and receives nourishment from 

 them; and in these villi blood- 

 vessels subsequently appear, con- ^ 

 iiected with the embryonic cir- 

 culation. The mucous membrane 

 of the uterus, rich in tubular 

 glands, and ordinarily covered 

 with ciliated columnar epithelium, 

 begins, even before the ovum 

 reaches it, to become thick and 

 spongy; it forms a growth which, 

 from being cast oft' at the birth of 



the child, is termed decidua; and Eig/147. DIAGRAM OF DE- 

 where the ovum is situated, this 

 rises up and invests it, forming 

 what is distinguished as the de- 

 cidua reflexa, while that which 

 lines the rest of the uterine 



CTDUA. a, Decidua vera; 

 b, decidua reflexa; c, de- 

 cidua serotina; d, ovular 

 space, with villi of chorion 

 round about; e, mucus 

 plugging the cervix. 



cavity is called decidua vera. A secretion of fluid like- 



