PHYLUM PLATYHELMINTHES 



257 



four cells is then separated off a small cell. The embryo at 

 this stage consists of eight cells, four large the megameres, and 

 four small the micromeres. The four micromeres increase rapidly 

 by division, and extend over the embryo, forming a layer, the 

 ectoderm, completely covering it in all parts except for a median 

 fissure, the Uastopcre, which runs along what is destined to become 

 le middle ventral line : this soon closes up. The ectoderm cells 



mes 



FIG. 205. Early stages in the development of a Polyclad. A, stage of four cells, of which 

 those lettered v and h correspond to the anterior and posterior portions of the body ; B to D, 

 later stages ; B and C, seen from above ; D, from the side ; E, earlier, and F, later stage of 

 epibolic gastrula, lateral view. ec. ectoderm ; en, endoderm ; mes. mesoderm ; o. en. and u. en. 

 upper and lower endoderm. (From Korschelt and Heider, after Lang.) 



soon develop a coating of cilia. The fourrmegameres then give off 

 four more small cells or micromeres. which increase in number by 

 division, and eventually form the middle layer or mesoderm of the 

 embryo. The megameres give off a number of additional micro- 

 meres, which form the endoderm layer, giving rise to the epithelium 

 of the intestine ; finally they become disintegrated, and their sub- 

 stance goes to nourish the cells of the developing embryo. The 

 process by which the germinal layers have become formed is, as in 

 the Ctenophora (p. 205), a process of epibolic gastrulation, and is to 



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