274 



ZOOLOGY 



separated off a small cell. The embryo at this stage consists of 

 eight cells, four large — the megameres, and four small — the 

 micromercs. The four micromeres increase rapidly by division, and 

 extend over the embryo, forming a la}^er, the ectoderm, completely 

 covering it in all parts except for a median fissure, the blastopore, 

 which runs along what is destined to become the middle ventral 

 line : this soon closes up. The ectoderm cells develop a 

 coating of cilia. The four megameres have previously given off 



mes 



Fio. 210.— 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.) 



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

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

 embryo. These extend over the surface below the ectoderm as 

 four mesodermal bands which subsequently fuse together to form 

 a continuous layer. The megameres give off a number of additional 

 micromeres which form the endoderm layer, giving rise to the 

 epithelium of the intestine ; finally the megameres become disinte- 

 grated, and their substance goes to nourish the cells of the develop- 

 ing embryo. The process by which the germinal layers have 



