364 L. DOXnASTER. 



development of the ventral ganglion. In the early stages 

 the ventral ectoderm does not differ from the dorsal, but 

 when the archenteric folds have not yet reached the pos- 

 terior end of the embrj-o, the ectoderm of the ventral 

 surface from just behind the head along the greater part of 

 the length of the body undergoes changes somewhat similar 

 to those described above in the somatic mesoderm. The 

 nuclei, which at first lie at the outer edges of the cells, sink 

 in till they reach their bases, and the cells at the same time 

 become larger than on the dorsal surface (fig. 8). There is 

 then a proliferation of nuclei along two bands lying in the 

 ventro-lateral areas, and at the same time the whole ectoderm 

 of the ventral half of the body increases in thickness (figs. 10, 

 11). In this way two bands of closely packed nuclei appear 

 just beneath the developing muscle-cells of the somatic 

 mesoderm, and when the latter are deeply stained the two 

 look very similar, but are always separated by a clear 

 dividing line. In the dorsal half of the body, and also 

 ventrally in the tail, this thickening and immigration of the 

 nuclei does not take place, and the cells retain longer their 

 original character. As development proceeds the ectodermal 

 cells along the mid-dorsal region become very thin, and con- 

 stitute a mere membrane separating the mesoderm from the 

 exterior ; but traced downwards from the mid-dorsal line the 

 ectoderm becomes steadily thicker, and is thickest along the 

 ventral middle line. 



V. Development of the Young Sagitta. 



The young Sagitta at the time of hatching differs very 

 markedly in structure from the adult, so that the term larva 

 might almost be used to describe it, especially as a rather 

 sudden change of structure takes place after a few days, 

 which may be compared with a metamorphosis. It is about 

 1 mm. in length, and so transparent as to be almost invisible, 

 but can be recognised by the naked eye as a minute shining 



