HABITS, liTO., OF CEREBRATULUS LACTEUS. 181 



in some sections the appearance of a double layer. They 

 retain their large blastula nuclei, and are quite active; they 

 also remain opaque, with the exception of those on the 

 anterior wall of the oesophagus, which are cleared in a 

 different manner. 



The Pilidium. 



The aboral ectoderm, which was a pavement epithelium 

 in the gastrula, has now become typically squamous, the 

 cells being strongly flattened. As the cells increase in 

 superficial area they lose the rounded form of the gastrula 

 and become more and more angular, until they finally assume 

 an irregular four- to six-sided form. The number of the cells 

 remains practically constant, except at the border of the oral 

 surface. 



These facts, compared with the increase of clear areas in 

 the gastrula, show that the pilidium grows largely, if not 

 wholly, by a flattening of the aboral cells. The aboral "um- 

 brella," as it is called, is simply a protective covering for 

 the young embryo, to be thrown away at the time of meta- 

 morphosis. Hence its cells, when once formed, remain until 

 final dissolution, simply flattening more and more to give the 

 necessary increase in size. Sometimes the flattening is carried 

 so far in the old pilidium that the body of the cell becomes 

 thinner than the nucleus, and the latter bulges out into the 

 body-cavity. Meanwhile the clear spaces have increased until 

 they occupy nearly all the cell, leaving but little cytoplasm 

 around the nucleus (fig. 72). The thinning and clearing of the 

 ectoderm, combined with the "glass-clear" gelatine between 

 ectoderm and entoderm, gives the pilidium great trans- 

 parency. But we should not expect the individual cells to 

 be held together very firmly, since they are in contact only 

 along their thin edges, and hence they pull asunder easily 

 under irritation. 



But the oral ectoderm takes an active part in the develop- 

 ment of the future Nemertean, and it is the only ectoderm 

 except the apical plate that shows karyokinetic figures. 



