ON THE DEVELOPMENT OF SAGTTTA. 377 



The trunk and tail as seen in sections now remain to be 

 described. On comparing a section through the trunk of a 

 first-day larva with that of an embryo, the only difPerences 

 are that the animal is more flattened dorso-ventrally, the 

 cells which will give rise to the ganglion are more marked 

 off from the epidermis, and the cells of the somatic mesoderm 

 are more collected together. Traced backward, the ventral 

 thickening dies out, and the animal becomes a narrower oval 

 in section, with epidermis one cell thick covering the meso- 

 derm dorsally and ventrally ; but laterally, where the fin is 

 beginning to form, it remains slightly thickened. A little 

 behind the ganglion the genital cells are seen pressed against 

 the septum which divides the mesodermal strands. Behind 

 them the animal gets gradually thinner, and more compressed 

 dorso-veutrally, but otherwise there is no important differ- 

 ence of structure. 



On the second day (PI. 21, fig. 31) the most noticeable 

 change is the reduction in size of the ventral ganglion. This 

 has taken place by a shrinking of the cells ; the nuclei remain 

 as before, aggregated veutro-laterally, but the cell protoplasm 

 which was so conspicuous between them and the epidermis 

 has now largely disappeared. In the mid-ventral region, 

 just below the alimentary canal, the rudiment of the fibrous 

 part {" Punktsubstanz ") of the ganglion is appearing. Some 

 of the nuclei {gang. nue. 2) in the ganglion are noticeably' 

 larger than the others. In some specimens, both alive and 

 in section, round cavities appear in the ganglion, one or two 

 on each side, but as they are rare and seem to occur irregu- 

 larly in the few cases where they are present, they are 

 probably pathological. A change has also taken place in the 

 mesoderm ; some of the nuclei of the somatic layer have again 

 moved inwards owing to the growth, between them and the 

 outer limit of the mesoderm, of a compact, rather faintly 

 staining mass of tissue, most of which gives rise to the 

 muscles {mus: c). Behind the ganglion region the only point 

 of importance is the further lateral outgrowth of the epidermis 

 into the rudiment of the fin as a sort of lateral fold. 



