160 HUMAN PHYSIOLOGY. 



result of nutritive changes in the foetus, to pass into the maternal blood, and 

 so to be carried to the various eliminating organs. 



Through the placenta also passes all the nutritious materials of the 

 maternal blood which are essential for the development of the embryo. 



At about the middle of gestation there develops beneath the decidual 

 membrane a new mucous membrane, destined to perform the functions of 

 the old when it is extruded from the womb, along with the other embryonic 

 structures, during parturition. 



DEVELOPMENT OF THE EMBRYO. 



Nervous System. The cerebro-spinal axis is formed within the me- 

 dullary canal by the development of cells from its inner surfaces, which 

 as they increase fill up the canal, and there remains only the central canal 

 of the cord. The external surface gives rise to the dura mater and pia 

 mater. The neural canal thus formed is a tubular membrane ; it termi- 

 nates posteriorly in an oval dilatation, and anteriorly in a bulbous extremity, 

 which soon becomes partially contracted, and forms the anterior, middle 

 and posterior cerebral vesicles, from which are ultimately developed the 

 cerebrum, the corpora quadrigemina, and medulla oblongata, respectively. 



The anterior vesicle soon subdivides into two secondary vesicles, the 

 larger of which becomes the hemispheres, the smaller, the optic thalami ; 

 the posterior vesicle also divides into two; the anterior becoming the 

 cerebellum, the posterior, the pons Varolii and medulla oblongata. 



About the seventh week the straight chain of cerebral vesicles becomes 

 curved from behind forward and forms three prominent angles. As devel- 

 opment advances, the relative size of the encephalic masses changes. The 

 cerebrum developing more rapidly than the posterior portion of the brain, 

 soon grows backward and arches over the optic thalami and the tubercula 

 quadrigemina ; the cerebellum overlaps the medulla oblongata. 



The surface of the cerebral hemispheres.is at first smooth, but at about the 

 fourth month begins to be marked by the future fissures and convolutions. 



The Eye is formed by a little bud projecting from the side of the 

 anterior vesicle. It is at first hollow, but becomes lined with nervous 

 matter, forming the optic nerve and retina ; the remainder of the cavity is 

 occupied by the vitreous body. The anterior portion of the pouch becomes 

 invaginated and receives the crystalline /ens, which is a product of the 

 epiblast, as is also the cornea. The iris appears as a circular membrane 

 without a central aperture, about the seventh week ; the eyelids are formed 

 between the second and third months. 



