'24 



plates, aic ili(Mi ovoi'p'i own liv ilic cells of the jiriiiiaiv 

 iH'todeiiu (PI. I\'.. fig-. 7) sunounding- them, so that at 

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

 a layer of secondary ectoderm covered externally hy the 

 layer of primaiv ectoderm, which has again become con- 

 tinuous {vf. n. IT., fig. 9). At the anterior end, a fifth 

 aiea of secondary ectoderm, the ])rol)oscidial plate, arises, 

 though it differs from the others in being formed by 

 (lelamiiiaiion, and not by sinking in (PI. IV., fig. 9, /')■/'.)■ 

 The five areas of the secondary ectoderm then spread out 

 and fuse with each other, forming a continuous coat which 

 lies directly beneath, and subse(|uently becomes entirely 

 separated from the 2:)rimary ectoderm. This secondary 

 ectoderm eventually forms the ectoderm of the adult. The 

 primary ectoderm is cast off later, degenerates, and is 

 \itili>ed as food material by the embivo. 



IJefore the fusion of the five secondary ectoderm plates, 

 however, two invaginations of the primary ectoderm are 

 formed on either side of the blasto})ore (PI. IT., fig. S, 

 forf/.). These later sink beneath the secondary ectoderm 

 between the cephalic and ventral plates of the latter, and 

 eventmxlly give lise to the ciliated canals of the cerebral 

 organs. In the process these invaginations lose their 

 communication with the exterior (PI. J\ ., fig. 10, coi-f/.), 

 but late]' a secondary opening is formed in each case at 

 the surface of the secondary ectoderm. 



Ai fhe time when the five plates of secondary ectoderm 

 are commencing to appear the fiist traces of the future 

 mesoderm are seen as cells l)udded off from both the 

 primaiy ectodeini and the endoderm (PI. IV., figs. 7, (S, 

 !)). AfttM- the estal)lishment of the secondary ectoderm 

 as a continuous layei-, these mesodeiin cells come to be 

 entirely enclosed within it. 



Meanwliile changes have been taking [)lace within the 



