380 L. DONCASTER. 



snbstanz." It also shows two kinds of nuclei in the ganglion ; 

 a few large ones^ faintly stained {gang, nuc. 2), and numerous 

 smaller ones (gang, nuc.) which stain more strongly. 



After the third and fourth days the changes are gradual 

 and less important. The coclom becomes completely clear, 

 and it also increases in size, while the body-wall becomes 

 thinner. The alimentary canal gets larger and more rounded, 

 so that the distinction between it and the dorsal and ventral 

 mesenteries becomes more obvious ; while in the tail, on the 

 other hand, the longitudinal septum becomes exceedingly 

 thin. The genital cells become tightly pressed against the 

 body-wall, and at the same time appear to be somewhat 

 reduced in size. The fins enlarge, and can be easily seen to 

 consist of a fold of the lateral epidermis enclosing a cnticular 

 supporting plate. In the genital region a mass of nuclei is 

 seen on the fin, which are those of the tactile sense-organ 

 present in that region [t. o.). The tactile organ mentioned 

 above, lying between the lateral and tail fins on each side, 

 appears as a mass of nuclei in the body-wall just at the 

 posterior end of the lateral fin ; the wall is so thin in this 

 region that it is impossible to determine whether they belong 

 to the ectoderm or mesoderm. The nuclei in these sense- 

 organs, like those of the ganglia, seem to stain more deeply 

 than those of the rest of the body. PI. 21, figs. 34, 35, 

 illustrate the above description, and ai'e taken iroxn sections 

 of a ninth-day larva; but after the sixth day hardly any 

 change seems to take place in the trunk and tail except the 

 further development of the alimentary canal and the forma- 

 tion of the anus. 



VII. Post-larval DEVELorMENx. 



In the last two sections the development has been followed 

 from the time of hatching up to the end of the second week, 

 beyond which period no young were ever kept alive. The 

 young at that stage has the essential structure of the adult, 



