24 PLATYELMINTHES. 



PLATYELMINTHES. 



(10) P. Hallez. Contributions a VHistoire naturelle des TurbeUaries. Lille, 1879. 

 (u) S. Max Schultze. Beitrcige z. Naturgeschichte d. Turbellarien. Greifswald, 



1851 



(1*2) C. Th. von Siebold. " Helminthologische Beitrage." Miiller's Archiv, 1836. 



(13) C. Th. von Siebold. Lehrbuch d. ver/jleich. Anat. d. wirbellosen I Mere. 



Berlin, 1848. 



(14) E. Zeller. "Weitere Beitrage z. Kenntmss d. Polystomen. Zeit. f. wiss. 



Zool., Bd. xxvn. 1876. 



[Vide also Ed. van Beneden] (No. i). 



This group, under which I include the Trematodes, Cestodes, 

 Turbellarians and Neraertines, has played a,n 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 Trematodes 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 protoplasm or not, it is quite certain that a little 

 lower down in the organ distinct cells are found, which have been 

 segmented off 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 con- 

 toured membranes, and containing nuclei. 



O 



As the yolk -cells grow older refracting spherules become 

 deposited in their protoplasm, which either completely hide the 

 nucleus, or render it very difficult to see. In the majority of cases the 

 entire cells forming the lining of the vesicles constitute the secretion 

 of the yolk-gland. They invest the ovum, and around them is formed 



