204 NEMERTEA. 



cavity. On the third day after impregnation the outermost cells of the 

 embryo become flattened and ciliated, and distinguished from the remain- 

 ing spherical cells of the embryo as the epiblast. With the appearance 

 of cilia a rotation of the embryo commences. On the fourth day the 

 embryo becomes oval, and at one of the poles the future anal pole a 

 separation takes place between the epiblast and the inner cells, giving rise 

 to the body cavity. In it are a number of loose oval cells, which soon 

 become stellate, and form a mesoblastic reticulum connecting the body-wall 

 and central cells of the embryo, which may now be spoken of as hypoblast. 

 The body-cavity increases in size, leaving at last the hypoblast and epiblast 

 united only at one point the oral pole at which, on the fifth day, a crown 

 of long cilia appears. The solid mass of hypoblast in the interior becomes 

 differentiated into an outer layer of cells the true glandular epithelium of 

 the alimentary tract and an inner core, the cells of which soon undergo 

 fatty degeneration, and serve as food-yolk. 



The later stages of development, and the formation of the proboscis, 

 etc., have not been worked out. 



General considerations. Of the types of larvae hitherto 

 found amongst the Nemertea, those with a metamorphosis, viz. 

 the Pilidium type and that of Desor, are to be regarded as the 

 primitive. But even in Pilidium there are evidences of a great 

 abbreviation in development. Pilidium itself is probably a more 

 or less modified ancestral form, while the peculiar development of 

 the Nemertine within it is to be explained as a very much short- 

 ened record of a long series of changes by which the Pilidium be- 

 came gradually converted into a Nemertine. The formation of 

 the body wall of the Nemertine by four epiblastic invaginations 

 is a remarkable embryological phenomenon, for which it is not 

 easy to assign a satisfactory meaning ; and it is probable that it is 

 merely a secondary process of growth similar to the formation of 

 imaginal discs in the larvae of Diptera (vide Chapter on Trache- 

 ata), which has had its origin in the abbreviation of the develop- 

 ment just alluded to. The development on the type of Desor is 

 clearly a simplification of the Pilidium type, and its peculiarities 

 are to be explained by the fact that the first larval form has no 

 free existence. The types without metamorphosis have no doubt 

 a development of a still more simplified character ; they are re- 

 markable however in presenting us, if the existing descriptions 

 are to be trusted, with examples of delamination and invagination 

 coexisting in closely allied forms. 



