DEVELOPMENT. 



97 



The collar nerve cord is delaminated from the ectoderm beneath the 



transient groove already mentioned. It remains however connected with 



the ectoderm throughout life at its 



front and hind ends. It appears to 



increase in length at the front and 



hind ends of the collar by invagination 



of the median ectoderm, by which pro- 



cess the continuous central canal pre- 



sent at these points is formed. 



In the subsequent changes the gill- 



slits increase in number from before 



backwards and the adult form is gradu- 



ally assumed (Fig. 71, C). A larval 



organ, constituting a suctorial tail for 



attachment, is developed and lasts for 



a short time ; it is placed at the hind 



end ventral to the anus. The noto- 



chord is formed as a dorso-median 



groove of the anterior part of the gut 



which becomes partially constricted off 



from before backwards to form a tube ; 



it later extends into the base of the 



proboscis. The pericardium is devel- 



oped from a solid mass of cells derived 



from the wall of the posterior part of 



the proboscis coelom. The first trace 



of the generative organs is seen when 



there are 10 gill-slits. Their exact ori- 



gin was not made out in this species, but from observations of other 



species they are probably mesodermal, though by some observers they are 



believed to be derived from 

 the ectoderm, with which in 

 any case they soon become 

 connected. So far they have 

 not been traced into connexion 

 with the coelom. 



The proboscis pore is first 

 indicated by a thickening of 

 ectoderm on the base of the 

 proboscis. This thickening 

 acquires a cavity which later 

 opens to the exterior and to 

 the proboscis coelom. The 

 collar pores arise by the per- 

 foration of thickened patches 

 of ectoderm in close connex- 

 ion with the opening of the 

 fi t ^ill..^ At the posterior 



FIG. 72. Diagram of a longitudinal sec- 

 tion through a larva of Dolichoglossus 

 koivalevskii (after Bateson. from K. and 

 H.). ci anterior (proboscis), cu middle 

 (collar), cm posterior (trunk) coelomic 

 diverticula, d enteron. 



32fev 



FIG. 73. Transverse section through the middle 

 part of the collar of a larva of Dolichoglossus kowa- 

 levskii which is at about the stage of Fig. 71 B 



IVVSKM wiuuii is at auuuu me stage ui xig. /i x> " . , 



(after Bateson, from K. and H.). Above is seen the end of the proboscis coelom, 

 dorsal ciliated groove, d enteron ; n rudiment of at the pomt w here the coelom 

 nerve cord ; cu collar coelom. _ ., , , , 



was connected with the gut in 



an earlier stage (Fig. 72), a proliferation of the coelomic wall occurs in 

 the dorsal middle line ; this causes a projection into the cavity which 



Z III. 



