THE STRUCTURE OF THE HUMAN^PLACENTA. 55 



II. OF THE VILLI OF THE CHORION. 



Without entering at present into the question as to the man- 

 ner in svhich the villi of the chorion take their origin, I may 

 state, that as soon as they are distinctly formed, they present a 

 structure which has to a certain extent been represented and 

 described by Raspail,* Seiler,f and others. 



The substance of the tufts consists of nucleated cells. These 

 cells are of different sizes. The smaller are situated, some in the 

 interior, others in the spaces between the latter. The cavities of 

 the larger cells are full of a granular fluid. The surface of the 

 tufts is bounded by a fine, but very distinct membrane, which, 

 when minutely examined, is seen to consist of flattened cells 

 united by their edges. 



The free extremity of each villus of the tuft is bulbous. The 

 cells which constitute this swelling are arranged round a central 

 spot. They are transparent and refractive, apparently from not 

 containing the same granular matter as the cells of the rest of 

 the villus and tuft. However short a villus may be, it invariably 

 presents a bulbous extremity, with the peculiar cellular arrange- 

 ment already described. Here and there, on the sides of the 

 stems of the tufts, swellings of a similar structure may be seen. 

 Each of these swellings is the commencement of a new villus or 

 stem, which, as it elongates, carries forward on its extremity the 

 swelling from which it arose. 



These groups of cells in the bulbous extremities of the villi of 

 the chorion, and in the swellings on the sides of their steins, are 

 the germinal spots of the villi. They are the active agents in 

 the formation of these parts. The villus elongates by the ad- 

 dition of cells to its extremity, the cells passing off from the ger- 

 minal spot, and the spot receding on the extremity of the villus, 

 as the latter elongates by the additions which it receives from it. 



The bulbous extremities of the villi of the chorion, are not 

 only the formative agents of these parts, but are also all along, 



* Raspail. " Chemie Organique" 

 f Seiler. 



