SIPUNCULID^ 359 



head-plate is not overgrown by the serosa, and consequently 

 a circular opening is always retained here. 



In consequence of a complicated process of growth, regard- 

 ing the details of which the i^eader must be referred to 

 Hatschek's descriptions, the at first median band-like 

 trunk-plate spreads considerably and grows out towards the 

 sides and then dorsal ly, finally to unite, at the termination 

 of the embryonic period, with the head-plate, which has 

 likewise enlarged somewhat. During this circumcrescence 

 of the internal parts by the trunk-plate, a change in the 

 position of the embryo takes place. The posterior part of 

 the trunk-plate had even before this turned toward the 

 animal pole, and thereby was in a position to supply the 

 dorsal part of the ectoderm (comp. Fig. 158 I) and E). In 

 the region of the blastopore, which closes, the oesophagus had 

 been formed from the ectoderm at an early period (Fig. 158 

 D, hi). This part also changes its position, for ib moves 

 more toward the apical plate, whereas the entodermal sac is 

 crowded further backwards (Fig. 158 E). As a result of 

 this, the position of the mesoderm is necessarily altered 

 (Fig. 158 (7, i>, and E). It is moved from the posterior 

 pai"t of the larva further forwards. Its cells have meanwhile 

 greatly multiplied, and two mesodermal bands have arisen 

 from it (Fig. 158 E, mes). The latter do not undergo a 

 segmentation ; on the contrary, a fissure makes its appear- 

 ance in them, which separates them into a splanchnic and 

 somatic layer. This ditferentiation is first noticeable in the 

 anterior part of the mesodermal bands, and proceeds from in 

 front backwards. 



The complete development of the embryo is reached by 

 the gradual closing of the permanent ectoderm. We saw 

 that the primitively band-like trunk-plate curved over 

 toward the dorsal side, and that its end moved toward the 

 apical plate. Since the band-shaped trunk-plate lies in the 

 median line, the embryo of such a stage appears in a 

 median section, almost enclosed by the permanent ectoderm 

 (Fig. 158 E) ; however, this is not actually the case, for, 

 although the ventral and dorsal parts of the trunk-plate also 

 spread out laterally, yet they remain separated on either 



