The Microscope. 7 



Above the intestine in the median hne of the posterior body- 

 half, lies the single ovary (Plate I, figures 1, dv., 11 a.; Plate II, 

 figure 21), the duct opening externally above the anal apertui*e. As 

 a rule but one egg is formed at a time, but not rarely two or more in 

 different stages of growth are observable in the same ovary. The 

 Bize of the egg is enormous in comparison with that of the animal, 

 not infrequently measuring nearly one-half the length of the entire 

 body. It is extruded rapidly and apparently with no inconvenience 

 to the animal, the oviduct and external orifice being surprisingly 

 expansile. The egg membrane is soft and flexible to a certain 

 degree, and the egg is often much compressed and variously dis- 

 torted in its passage, remaining so for several seconds after its 

 extrusion. It is not permanently attached to any submerged object, 

 but is dropped wherever the Chsetonotus may be feeding, and is left 

 to take its chances of being devoured by the Turbellarian worms or 

 other innumerable enemies abounding in the superficial ooze. The 

 eggs of many species are, however, protected by an armor of spines, 

 papillae or stiff hairs. Yet others are either quite smooth, or only 

 granularly roughened. It is a curious fact in this connection, that 

 one side of the egg membrane is always unarmed, the spines and 

 other outgrowths being confined to the ends and one border. And 

 no less interesting is the fact that the same species may deposit eggs 

 whose CKmamentation differs as greatly as a network of raised lines 

 differs from pentangular hollow papillae, or long spines with tri- 

 radiate or four-parted ends. I at first supposed that each species 

 deposited an egg with characteristic external markings, but a little 

 further observation soon dispelled that pleasing illusion. 



It would not be very difficult for two observers, working to- 

 gether, to follow the development of the embryo from the moment 

 when the egg is extruded to the escape of the matured creature. 

 But for one working alone it is hardly possible. The shortest time 

 between the periods just mentioned is, so far as I have personally 

 ascertained, about thirty hours. This is in the egg of Ch. spinosu- 

 lus (Plate I, fig. 2). To sit at the microscope for thirty hours con- 

 tinuously would be no small task. 



The internal changes take place rapidly in the earlier stages, 

 and begin immediately after the egg enters the water. The 

 contents at once become more and more granular, and soon assume 

 a spherical form in the middle of the egg', the conspicuous nucleus 

 divides, the parts separate, and the egg-contents undergo what seems 

 to be total segmentation. In Ch. loricatus (Plate II, fig. 16) this 

 stage is reached in about three hours. 



