PLATHELMINTHES 



163 



intestine had become useless, owing to the metamorphosis of a portion of 

 the entoderm cells into food-substance, and another part had been com- 

 pelled to move into its place, could this process give rise to a distmct 

 mesoderm. 



The further development of the embryo of Biscocelis con- 

 sists first of a complete overgrowth on the part of the 

 ectoderm and the resulting- closure of the blastopore. The 

 elements of the ectoderm become more like an epithelium, 



Fig 76.-.1 to C (after A. Laxg). -4 and B. e.nbryos of Discocdis Ugpna, seen 

 from the ventral side; C. median longitudinal section through ^^^^^^ f^^^^^ ° 

 St,yodu.s rü.-dium. EC, ectoderm ; En, remains of the '^''^^^''^^' 'f'^^^ZZ 

 of disintegration ; Ec, fundament of the excretory organ (?) -D branches of the 

 intestine and (in C) intestinal epithelium ; Mcs, mesoderm ; N, fundament of the 

 central nervous system. 



their cilia stronger and more dense. A change in the outer 

 form now takes place, the aboral pole being pushed forward, 

 the oral backward. The anterior end is distinguished by 

 the appearance of the first pair of eyes, which here arise as 



