DEVELOPMENT OF THE GENERATIVE ORGANS. 721 



Judging from the above description, it appears that the 

 intracellular development of germs is a widely-spread pheno- 

 menon, and that germ-ova originate by true endogenous cell- 

 formation ; and this view is still further borne out by the 

 endogenous development of spermatozoa. 



Later authors have rather confined their observations to the 

 manner in which the nuclei multiply ; but I think it is clear 

 that the continuous layer of protoplasm in which the germ- 

 ova develop is, as Van Beneden suggests, either a single cell 

 with many nuclei, or consists of a group of cells, the individual 

 limits of which have not been distinctly observed probably 

 the result of the processes employed for their preservation. 



k. Some General Remarks on the Similarity of the Generative 

 Function in Insects and Trematodes. 



Although the Trematoda are always hermaphrodite, and 

 the Insecta are invariably unisexual, there is a remarkable 

 similarity in the general disposition of the female generative 

 organs of the two groups, especially when the generative 

 organs of the highest Trematodes, the Rhabdocrelian Turbel- 

 laria, and Insecta are compared. 



The diagrammatic representation of these organs in a Rhab- 

 doccelian (see next page) would serve as a representation of 

 the same organs in a female Insect. In both cases there is a 

 bursa and a receptaculum seminis, or spermatheca ; in both 

 the vitelline glands apparently correspond with the testes of 

 the male ; and in both, if I am right in my conclusions, there 

 is a distinct germ-gland. Moreover, the germ-gland in the 

 Turbellarian is very similar to the parovarium of the 

 Insect. 



There are, of course, points of dissimilarity, the most im- 

 portant of which is the structure of the vitellogenous gland. 

 This in the Turbellarian is an ordinary racemose gland, the 

 ducts and follicles of which are continuous ; but this condition 

 is by no means uncommon in the Arthropoda, and is the form 

 usually ascribed to the ovaries of Insects. 



