GENERATION AND DEVELOPMENT. 265 



processes, the so-called villi of the chorion (Figs. 422, 423), which give 

 it a rough and shaggy appearance. At first only cellular in structure, 

 these little outgrowths subsequently become vascular by the development 

 in them of loops of capillaries (Fig. 423); and the latter at length form 

 the minute extremities of the blood-vessels which are, so to speak, con- 



FIGS. 422 and 423 (after Todd and Bowman), a, chorion with villi. The villi are shown to be best 

 developed in the part of the chorion to which the allantois is extending; this portion ultimately be- 

 comes the placenta; 6, space between the two layers of the amnion; c, amniotic cavity; d, situation 

 of the intestine, showing its connection with the umbilical vesicle ; e, umbilical vesicle ; /, situation 

 of the heart and vessels; g, allantois. 



ducted from the foetus to the chorion by the allantois. The function of 

 the villi of the chorion is evidently the absorption of nutrient matter for 

 the foetus; and this is probably supplied to them at first from the fluid 

 matter, secreted by the follicular glands of the uterus, in which they are 

 soaked. Soon, however, the fcetal vessels of the villi come into more 

 intimate relation with the vessels of the uterus. The part at which this 

 relation between the vessels of the foetus and those of the parent ensues, 



FIG. 424. 



is not, however, over the whole surface of the chorion : for, although all 

 the villi become vascular, yet they become indistinct or disappear except 

 at one part, where they are greatly developed, and by their branching give 

 rise, with the vessels of the uterus, to the formation of the placenta. 



