LINETJS. 265 



plates, are then overgrown by the cells of the primary 

 ectoderm (PI. IV., fig. 7) surrounding them, so that at 

 these four areas the ectoderm becomes two-layered, viz., 

 a layer of secondary ectoderm covered externally by the 

 layer of primary ectoderm, which has again become con- 

 tinuous (cf. PI. IV., fig. 9). At the anterior end, a fifth 

 area of secondary ectoderm, the proboscidial plate, arises, 

 though it differs from the others in being formed by 

 delamination, and not by sinking in (PI. IV., fig. 9, prp.). 

 The five areas of the secondary ectoderm then spread out 

 and fuse with each other, forming a continuous coat which 

 lies directly beneath, and subsequently becomes entirely 

 separated from the primary ectoderm. This secondary 

 ectoderm eventually forms the ectoderm of the adult. The 

 primary ectoderm is cast off later, degenerates, and is 

 utilised as food material by the embryo. 



Before the fusion of the five secondary ectoderm plates, 

 however, two invaginations of the primary ectoderm are 

 formed on either side of the blastopore (PL IV., fig. 8, 

 corg). These later sink beneath the secondary ectoderm 

 between the cephalic and ventral plates of the latter, and 

 eventually give rise to the ciliated canals of the cerebral 

 organs. In the process these invaginations lose their 

 communication with the exterior (PI. IV., fig. 10, corg.), 

 but later a secondary opening is formed in each case at 

 the surface of the secondary ectoderm. 



At the time when the five plates of secondary ectoderm 

 are commencing to appear the first traces of the future 

 mesoderm are seen as cells budded off from both the 

 primary ectoderm and the endoderm (PI. IV., figs. 7, 8, 

 9). After the establishment of the secondary ectoderm 

 as a continuous layer, these mesoderm cells come to be 

 entirely enclosed within it. 



Meanwhile changes have been taking place within the 



