60 

 of tme durinc: which the evbryo remains in the free-swim- 

 niing planula stage is variable; but as a rule by the ti-:Te it 

 is about forty-eight hours old, it begins to sink tov;ard 

 the botto.- of the aquariun^, and to svini less rapidly. After 

 the sniral sv;iir,rr.ing rriovements are lost, zhc wlanula is 

 capable of gliding along the bcttoir. of the dish for some 

 time. Finally the notion ceases altogether and the larva loses 

 its cilia and is ready for ::ttach:ient. This stage of de- 

 velopment is reached under favorable conditions about forty- 

 eight to fifty hours after the eggs have been laid. 



The rlanula is very opaque, and thus it is i-mossible 

 to rriake out anything about its internal structure in study- 

 ing the living forr^.s. Specimens in various stages oi de- 

 velonr.ent were preserved and sectioned for the study of 

 cellular structure. The descriDton of this structure v;ill 

 be given in connection with the forration of the ger- layers. 



Brooks describes and figures an ectodernr.al invagination 



