EVOLUTION IN A DETERMINATE LINE. IO/ 



the egg was contained in but a single region (trunk region) of 

 the capsule. In a word, the capsule is destined to fit the shape 

 of the embryo when full grown as accurately, to seek for a 

 comparison, as the mummy case its mummy. 



III. /// Permanence.- -The development of the young Chi- 

 wizra is slow, taking many months, perhaps a year in hatching. 

 The capsule is adapted to future conditions in its shape and tex- 

 ture, enabling it to resist wear and tear as well as the softening 

 action of the water. Its " life," in other words, corresponds with 

 the duration of the embryo's period of incubation. 



IV. In Attachment. - -During its long service the capsule is 

 insured against displacement and attendant injury by means of 

 specialized structures. These include a definite stalk, long, 

 tough, springy, and a disc- or bulb-shaped terminal, capable of 

 durable attachment. 



V. In Position. Experiments with capsules containing eggs 

 show that when there is even a slight movement of water the 

 capsules, as they lie on the bottom, assume a position with the 

 keel side upward. This is the side which is dorsal in the orien- 

 tation of the embryo and which bears the lid through which the 

 fish later escapes. Furthermore, the capsule is provided with a 

 vertical and a pair of lateral expansions which, if a current of 

 water is present, cause it to rotate like a weather-vane in the di- 

 rection of the oncoming stream. By this means the young fish 

 is adjusted to its respiratory current and at the same time pro- 

 tected against the injuries which would follow the continual 

 dislodgment of the capsule. 



VI. /// its Orientation witJi Respect to the Growing Embryo. The 

 germinal disc and early blastoderm in about fifty capsules exam- 

 ined occupied a constant position with reference to the capsule, 

 appearing on the keeled side, and when the young embryo could 

 be determined the head always pointed in the direction of the 

 large end, /. c., the operculate end, of the case. The same posi- 

 tion is maintained in later stages, the head directed toward the 

 large end of the capsule, the tail gradually invading the narrow 

 sheath-like end. Symmetry is maintained with reference to the 

 sagittal plane of the capsule. 



VII. /// its Arrangement Enabling the Imprisoned Embryo to 



