42 MORPHOLOGY OP 



probably pass directly through it. As will be shown in the 

 next paragraph its distribution agrees with this view. 



In the account of the general development the central 

 nervous system was shown to have arisen chiefly by a solid 

 delamination from the skin, added to which its anterior, and 

 to some degree its posterior, ends are being continually 

 invaginated as growth continues, so that each end is tubular. 

 This tubular form results not so much from the longitudinal 

 closure of a tube as from a forward and backward growth of 

 skin at the extremities of the delaminated cord. Soon after 

 delamination histological differentiation occurred between the 

 upper cellular and lower fibrous parts of the cord. While 

 this was proceeding (2, g. s.) fibrous tissue was deposited to 

 form the ventral cord at the point of this structure, which 

 was most anterior (viz. the back of the collar). While this is 

 proceeding the deposition of similar tissue in the region of the 

 dorsal cord commences at the posterior attached end of the 

 central nervous system. Next, the deposition of fibrous tissue 

 extends itself forwards on to the proboscis, being first laid down 

 in the dorsal middle line of the proboscis stalk {o. figs. 34 and 

 35, pkt.). On the appearance of the atrial fold the ventral 

 and dorsal cords become united by a fibrous ring in the inner 

 angle of the fold. This ring, therefore, may be supposed to 

 bring up the fibres from the ventral cord to the central 

 nervous system, which it enters at its posterior end, together 

 with the dorsal cord (v. diagram, fig. 65). 



The greatest concentration following upon these occurs in 

 the skin of the base of the proboscis. In the larva with four 

 gill-slits (fig. 99, P. rg.) it is already well marked. Concen- 

 trations are formed in the line of the gill-slits (figs. 72a and 

 104), and slight fibrous anastomosing tracts run irregularly, 

 following the line of the wrinkles from both the dorsal and 

 ventral cords. These wrinkles taper towards both the cords 

 and are permanent, being, in fact, limiting lines between 

 patches of glandular cells. 



Now, all these tracts of fibres are thickened as they approach 

 the central nervous system, and dwindle peripherally. If this 



