NEMATODA 229 



thick vitelline membrane. Then it forms a strong and resist- 

 ing 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 

 Kiichenmeister has represented under the form of a small fili- 

 form 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 Claparede 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 inconvenience. In a few hours the 

 transformations will commence and go on continuously until the 

 vermiform condition is attained. If, as remarked in my 



