252 Dn. m'intosu on tue development 



tract existing at each end remains closed by firm contraction of 

 the circular muscular fibres of the body-wall ; but by-and-by new 

 cell-growth occurs at both extremities, especially the anterior. 

 At the latter end the parts that are firmly contracted by the 

 primary muscular spasm gradually become more or less conso- 

 lidated by a cicatrix. This new growth steadily increases in 

 bulk, marking out the anterior end of the fragment, even in 

 its early stages, by its conspicuous paUor. The appearance of the 

 anterior end of a specimen, probably about three weeks after 

 rupture, is shown in fig. 1, Plate VII. The head is represented 

 by the pale sprouting mass in front of the digestive tract, and 

 there is no further different iation of organs than the separation of 

 the exterior cutaneous elements from the inner mass, and the 

 ciliated aperture («) leading into the chamber of the proboscis. 

 The three contractile circulating channels of the body course 

 forwards to the pale developing region, and apparently com- 

 municate with each other without passing into it ; they are 

 connected by the usual transverse branches throughout their 

 course. The posterior end of the fragment shoots out into a 

 conical tail (fig. 2, Plate VII.) with a well-formed anus (z) in 

 its usual position, and through which a prolapsus of the wall 

 of the digestive tract occasionally occurs under pressure, or an 

 escape of one or more gregariniform parasites. 



In the next stage (fig. 3, Plate VII.) the anterior end has as- 

 sumed a more conical form, and a greater differentiation of organs 

 has occurred. The cutaneous elements are distinctly marked 

 off", and a miniature proboscis («') occupies its sheath, both 

 springing from a point at some distance behind the tip of the 

 snout, and corresponding to the commissure of the developing 

 ganglia (h), which, however, are scarcely apparent. The probos- 

 cidian sheath contains a clear fluid and granules, which now 

 and then distend the cavity in front as in the figure. The pro- 

 boscis (a') is quite free posteriorly. The cephalic fissures are indi- 

 cated on each side by slight superficial grooves, very strongly 

 ciliated. Besides the faint outlining of the ganglia, which spring 

 from the anterior ends of the nerve-trunks, the cephalic pits and 

 glands (m) are likewise indicated. The circulation in the vessels 

 extends only to the posterior border of the white snout. The 

 digestive tract presents no subdivision into regions. 



A more advanced condition of the head is found after two or 

 three months (fig. 4<, Plate VII.). The snout is very much 



