EMBRYOLOGY. 271 



Through the placenta also passes all the nutritious materials of 

 the maternal blood which are essential to 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 

 medullary canal by the development of cells from its inner sur- 

 faces, 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 terminates 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 the 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 development 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 lens, which 



