ON PARASITES. 31 



nourished at the expense of the umbilical vesicle, a suitable communi- 

 cation is requisite for the purpose. Accordingly the omphalo merentine 

 vessels are produced to carry the needful supplies from the vesicle to the 

 embryo. But soon this source of nourishment becomes exhausted, and 

 the now enlarged and more developed foetus needs a more abundant 

 supply than the umbilical vesicle could afford at any time ; moreover 

 an apparatus is required to remove from it effete matter, for there is a 

 destructive metamorphosis of tissue in development as in maintenance. 

 To meet this want, there buds out from the cloacal extremity of the 

 alimentary canal, which by this time has assumed shape, the allantois 

 which continually and rapidly enlarging fore and aft and laterally 

 inside the chorion, and approaching nearer and nearer to that outer 

 envelope, finally comes into immediate contact with it. This accom- 

 plished, it insinuates its terminal capillary loops of bloodvessels, which 

 by arteries and veins are perpetually in anastomatic connection with the 

 fcetal vessels, into the villosities of the chorion. These in time being 

 insinuated into the continually enlarging and divaricating tubular 

 follicles of the hypertrophied uterine mucous membrane, an intimate 

 relation is established between the maternal vessels on the one hand, 

 and the fcetal ones on the other. Whilst the foetal and maternal 

 vessels are brought into the closest relation by this wonderful contri- 

 vance, no anastomosis takes place between them. That would defeat 

 the whole matter. Now by endosmose nutriment is transferred 

 through the intervening walls from the maternal vessels to the foetal 

 ones. Of all contrivances in nature for the accomplishment of any 

 purpose this is among the most perfect. Upon it alone the defenders 

 of our inherent vital force might take their stand. At the same time 

 it is seen that material conditions and their affections concur in the 

 final result. 



This subsidiary circulation becomes the placental, and continues 

 until birth. Then the embryo no longer needing its use, it is cast off, 

 and a new circulation established that holds during the rest of life. 

 Now compare this apparatus with the caudal vesicle of the cystic 

 worm. If it were required to construct a nutrient apparatus in which 

 a tsenoid scolex should safely develope in the interior of a serous 

 cavity or in cellular tissue, could a more appropriate one be made than 

 that same caudal vesicle ? There could not. It completely meets its 

 requirements as a protecting and nutrient apparatus, and no more can 

 be demanded. 



