XVII 



PROTOCHORDATA 



583 



The pericar dial vesicle originates as a solid outgrowth of the posterior 

 horn of the proboscis-cavity ; and the dorsal nerve cord is delaminated as 

 a solid strip of ectoderm, into which canals extend, subsequently from 

 tli.r. anterior and posterior ends of the collar where it remains in 

 connection ivith the ectoderm. 



Davis' account of the embryology of D. pusillus (1908) agrees 

 in many points with Bateson's account of the development of D. 

 koiualevskii, which we have just summarized. The embryo hatches 

 out and begins free life when the collar region is delimited and one 

 pair of gill-pores has appeared. 



Owing to the fact that the mud was of a more cohesive kind 

 than that in which D. koivalevskii lives, it was possible, at low tide, 

 to take spadefuls of mud containing the 

 burrows intact, and when these burrows 

 were broken open the fertilized eggs were 

 seen clinging to one side of them. A 

 mixture of corrosive sublimate and acetic 

 acid, osmic acid, and Lo Bianco's chrom- 

 osmic mixture, were used for preserving 

 the embryos. The embryos and larvae, 

 which were removed from the burrows, 

 lived and completed their development in 

 vessels of clean water in the laboratory. 



When the larvae are first hatched 

 they swim in spirals and rise to the 

 surface, but soon tire and drop to the 

 bottom (Fig. 426). These swimming 

 efforts at first occur at regular intervals, 

 but the intervals become gradually longer 

 and longer, and finally the larvae only FK;. 426. The larva of 

 glide over the bottom. Soon muscular giossus pwaim in the act of 

 movements are observable in the proboscis, 

 and with the loss of cilia the burrowing 

 life of the adult is begun. These 

 spasmodic upward movements, however, 

 give opportunities to the tidal current to waft the larvae far from 

 their birthplace. 



The principal points of interest in the development of D. pusillus, 

 as brought out by Davis, are as follows: (1) The cleavage is not 

 quite regular, for, as in the egg of Amphioxus, the upper four blasto- 

 meres of the 8 -cell stage are smaller than the lower four, and 

 there is a cleavage pore ; that is to say, that the incipient blastocoele, 

 formed by the separation of the blastomeres, opens above and below. 

 (This also occurs in the egg of Amphioxus.') (2) Both middle and 

 posterior pairs of coelomic cavities arise as outgrowths of the anterior 

 body-cavity, so that, as in Echinoderms, there is one anterior 

 e vagina tion. of the archenteron which gives rise to all the body- 

 cavities (Fig. 427). 



escaping- from the egg-membrane. 

 (After Davis. ) 



"/>. apical jilafr ; <W, collar region ; 

 e.m, c'uu-iiii'inljiaiii' ; t.ti-, tclotrocli. 



