30 PLATYELMINTHES. 



Turbellarians and Nemertines, has played an important part in 

 all controversies relating to the nature and composition of the 

 ovum. The peculiarity in the development of the ovum in 

 most members of this group consists in the fact that two organs 

 assist in forming what is usually spoken of as the ovum. One 

 of these is known as the ovary proper, and the other as the 

 vitellarium or yolk-gland. In the sequel the term ovum will be 

 restricted to the product of the first of these organs. In Trema- 

 todes the ovary forms an unpaired organ directly continuous 

 with an oviduct into which there open the ducts from paired 

 yolk-glands. 



The ovary has a sack-like form and contains in some 

 instances a central lumen (Polystomum integerrimum). At the 

 blind end of the organ is placed the germinal tissue. This part 

 is, according to the accounts of the majority of investigators, 

 formed of a polynuclear mass of protoplasm not divided into 

 distinct cells. Whether it is really formed of undivided proto- 

 plasm or not, it is quite certain that a little lower down in the 

 organ distinct cells are found, which have been segmented oft' 

 from the above mass, and are formed of a large nucleus and 

 nucleolus, surrounded by a delicate layer of protoplasm. These 

 cells are the young ova. They usually assume a more or less 

 angular form from mutual pressure, and, in the cases where the 

 ovary has a lumen, constitute a kind of epithelial lining for the 

 ovarian tube. They become successively larger in passing- 

 down the ovary, and, though in most cases naked, are in some 

 instances (Polystomum integerrimum) invested by a delicate 

 vitelline membrane. Eventually the ova pass into the oviduct 

 and become free ; and at the same time assume a spherical 

 form. 



In the oviduct the ovum receives somewhat remarkable 

 investing structures, derived from the organ before spoken of as 

 the yolk-gland. The yolk-gland consists of a number of small 

 vesicles, each provided with a special duct, connected with the 

 main duct of the gland. Each vesicle is lined by an epithelium 

 of cells provided with doubly contoured membranes, and con- 

 taining nuclei. 



As the yolk-cells grow older refracting spherules become 

 deposited in their protoplasm, which either completely hide the 



