No. 2.] STOLOxNIZATION IN AUTOLYTUS VARIANS. 307 



As may be seen in the earlier stages of the development of the 

 nerve ring, the nerve fibers composing this ring make their 

 appearance as downward growths from the medullary substance 

 of the brain, while the fibers of the ventral cord remain passive. 



The remaining important stage in the development of the 

 head consists in the shifting of the anterior eyes from the 

 dorsal surface of the head to a position almost directly ventral 

 to the posterior eyes. The change in position of the eyes first 

 becomes apparent in a stolon a little younger than stolon 4. 

 At this stage a transverse section in a plane with the anterior 

 eyes will reveal a mid-dorsal area of rapid growth, extending 

 from this plane forward to the anterior part of the head. In a 

 transverse section of the head of a stolon, corresponding in 

 development to that of stolon 5 (PI. XIV, Fig. 28), this growth 

 has already been sufficiently great to carry the anterior eyes 

 (e.a) from their original dorsal position to a lateral ventral 

 position, and the lens (/.), instead of being directed upward, is 

 directed almost directly downward. In later stages this process 

 will be continued by the active increase of the brain cells in the 

 mid-dorsal region (cn.d.) until the anterior eyes occupy a posi- 

 tion almost ventral to the posterior eyes, the complete location 

 of the eyes taking place immediately after the separation of the 

 stolon. 



The development of the eyes in the stolon of Autolytus 

 varians is so similar to the development of the eyes in the 

 embryo, and so identical with that described by Andrews (13), 

 that I shall not enter into a description of this process. 



Development of the Mesodermal Structures. 



The development of most of the mesodermal structures of 

 the stolon is also identical with the development of similar 

 organs in the embryo, and hence needs but little mention here. 

 The mesodermal tissue of the embryonic region, even in the 

 youngest segment, is not uniformly embryonic throughout, 

 for at this stage there may already be distinguished in the 

 dorsal regions indistinct bands of sparsely scattered muscle 

 fibers, which are a continuation of the dorsal musculature of 



