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225 



105 Ato C). In this way there arose under the ectoderm four 

 cell-plates, corresponding to the prostomial and metastomial 

 discs of the pilidium, but distinguished from them by the 

 fact that they are not composed of two layers, but of only 

 one cell-layer (Fig. 106). When subsequently they grow 

 around the embryonic intestine, there is thus formed only 

 one cell-layer, the body- wall. The amnion is wanting (Fig. 

 107 B). The body, of course, is still surrounded by the 

 larval skin, the original ectoderm. At the place where the 



Fig. 107. — A, gastrula stag-e of Linevs ohscurus, seen from the side ; B and C, 

 older embryos of Lineus seen from the ventral surface (after Ba.beoi3, from Bal- 

 four's Comparative Embryology) ; ae, archenteron ; cs, lateral organs ; Is, larval 

 skin ; m, mouth ; me and ms, mesenchyma ; pr.d, prostomial disc ; po.i, meta- 

 stomial disc; pr. proboscis; st, stomach. 



prostomial discs come together, the proboscis arises as a 

 solid ingrowth of the ectoderm, which subsequently becomes 

 hollow (Fig. 107 C). 



HuBRECHT describes a fifth plate, derived from secondary ectoderm, in 

 addition to the four plates found by Barrois (Fig. 106 S). It is said to 

 be formed on the dorsal side of the embryo, but in a different manner 



K. H. E. Q 



