748 . DEVELOPMENT OF THE IMPREGNATED EGG. 



tributed to the alimentary canal, pass out along the canal of communi- 

 cation to the chorion and ramify over its surface. The embryo at this 

 time has reached such an activity of growth that it requires to be sup- 

 plied with nourishment by means of vascular absorption, instead of the 

 slow process of imbibition hitherto taking place through the compara- 

 tively structureless villi of the chorion. The capillary bloodvessels, with 



which the chorion is supplied, begin to pene- 

 Fig. 262. trate the substance of its villosities. They 



enter the base or stem of each villus, and. 

 following the division of its compound rami- 

 fications, reach the rounded extremities of 

 its terminal offshoots. Here they turn upon 

 themselves in loops (Fig 262), and retrace 

 heir course, to unite finally with the venous 

 runks of the chorion. 



The villi of the chorion are, accordingly, an- 

 alogous in structure and function to those of 

 the intestine ; their power of absorption cor- 

 responding with the abundance of their rami- 

 fications, and the extent of their vascularity. 



Extremity of a VILLOSITY ,,,. , , -, , ,, , . 



OP THE CHORION, magnified Hie bloodvessels of the chorion, further- 

 iso diameters; showing the ar- mO re, are all derived from the abdomen of 



rangement of bloodvessels in its . _ ' . ,, . 



interior. the foetus ; and all substances absorbed by 



them are transported to the interior of the 



body, and used for the nourishment of its tissues. The chorion, there- 

 fore, as soon as its villi and bloodvessels are completely developed, be- 

 comes an active organ in the nutrition of the fo3tus ; and constitutes 

 the only means by which new material is introduced from without. 



The third event of importance in the history of the chorion is that 

 after being at first uniformly shaggy throughout, it afterward becomes 

 partially bald. (Fig. 260.) This change begins about the end of the 

 second month, commencing at a point opposite the insertion of the 

 foetal bloodvessels. The villosities of this region cease growing; and 

 while the entire egg continues to enlarge, they fail to keep pace with 

 the progressive expansion of the chorion. They accordingly become at 

 this part thinner and more scattered, leaving the surface of the chorion 

 comparatively bald. This baldness increases in extent, spreading over 

 the adjacent portions of the chorion, until at least two-thirds of its sur- 

 face have become nearly or quite destitute of villosities. 



At the opposite pole of the egg, namely, that which corresponds with 

 the insertion of the foetal bloodvessels, the villosities of the chorion, 

 instead of becoming atrophied, continue to grow ; and this portion be- 

 comes even more shaggy and thickly set than before. The consequence 

 is that the chorion afterward presents a different appearance at different 

 parts of its surface. The greater part is smooth; but a certain portion, 

 constituting about one-third of the whole, is covered with a soft, spongy 

 mass of long, thicks-set, compound villosities. It is this thickened 



