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CHAPTER YI. 



THE DEVELOPMENT OF THE HUMAN 

 EMBRYO. 



PRELIMINARY ACCOUNT. 



The human embryo is developed from an egg, which, like that 

 of other animals, is a single nucleated cell, derived from the 

 peritoneal cells forming the outermost layer of the ovary. The 

 human egg, or ovum, measures 0'2 mm. in diameter, i.e. is 

 rather less than double the diameter of the ovum of the rabbit. 



The ovum, when ripe, is discharged from the ovary, and is 

 taken up by the open mouth of the Fallopian tube or oviduct, 

 down which it travels to the uterus, where it remains during 

 the rest of the period of development. Prior to the arrival of the 

 ovum, the mucous membrane lining the uterus undergoes impor- 

 tant changes, and gives rise to a special layer, the decidua, to 

 which the ovum is attached and in which it becomes embedded. 

 During gestation the cavity of the uterus gradually becomes 

 filled up by the growth of the embryo and of its inclosing 

 membranes. 



As the ovum is of very small size, the nutriment at the 

 expense of which development takes place must be obtained 

 from without. This is effected, as in the rabbit, by means of 

 the placenta, an organ in which the blood-vessels of the embryo 

 and those of the wall of the uterus are brought into extensive 

 contact, so that free interchange of contents can take place 

 through their walls. 



The process of fertilisation of the human ovum, and the early 

 stages of development have not yet been seen ; and of specimens 

 showing the first formation of the embryo only a very limited 

 number have been obtained, and very few of these in fit 

 condition for microscopical investigation. Of later embryos, 

 numerous specimens have been examined and described, and 



