Many years back (1863) I pointed out that the most advanced eggs whilst still within the body of the pregnant female contained tadpole-shaped embryos, and about the same time the fact was noticed by Claparède. In his beautiful and scholarly memoir, ‘De la formation et de la fécondation des œufs chez les vers Nématodes,’ he wrote concerning the ova as follows:--“The egg, which exhibits the form of a very narrow disk in the ovary, acquires the shape of an elongated ellipsoid in the oviduct, and at the surface differentiates itself into a very thick vitelline membrane. Then it forms a strong and resisting chorion, which imparts to the egg an outline similar to that of a bridge’s span. It has an oval figure flattened at one of its sides. This chorion is very fragile; it frequently gives way under slight pressure from the thin plate of glass which covers the object. It extends itself considerably under the action of acetic acid, acquiring a size three or four times greater than that of the egg. The constitution of this chorion is perfectly identical in the eggs both before and after impregnation. It is, nevertheless, easy at first sight to know whether or not we have to deal with a fecundated egg. In the impregnated females the uteri are filled with thousands of ova, each one of which encloses an embryo already well formed. The ventral surface of the embryo and the tail are, without exception, applied to the flattened side of the egg. The embryo is very broad in the body, and occupies all the interior space. An embryo such as Küchenmeister has represented under the form of a small filiform worm folded on itself, and only occupying a very small part of the cavity of the egg, is never to be seen. In the non-fecundated females, on the other hand, the uteri are filled with eggs, which, instead of the embryo, enclose a non-segmented yolk furnished with a large germinal vesicle. This vesicle is not visible so long as the eggs have the form of thin disks; it only shows itself when the eggs begin to acquire an elliptical form in the oviduct. It is, however, probable that this vesicle is the same which was originally visible in the ovary.” The chorion itself is homogeneous, but in an allied species (_Oxyuris spirotheca_) Gyoery and Claparède found that this egg-covering consists of spirally-coiled bands resembling the tracheal spiral fibre of an insect. Under suitable conditions the tadpole-shaped embryos rapidly assume a vermiform character. The investigations of Leuckart have shown that “one only needs to expose the eggs to the action of the sun’s rays in a moistened paper envelope when, at the expiration of five or six hours, the tadpole-shaped embryos will have already become slender elongated worms.” According to Heller, the simplest way to rear the vermiform stage of Oxyuris is to put a number of the eggs in a glass tube filled up with saliva. The tube should then be placed in the arm-pit, in which situation it can be carried about with little