PLATYELMINTHES. 203 



epiblast, and then fuse with the syncytium within the blastosphere. Dieck 

 finds that in Cephalothrix the invaginated mass simply vanishes. 



Barrois' statements about the fusion of the syncytium derived from the 

 epiblast cells with the invaginated cells must be regarded as very doubtful. 

 The formation of the germinal layers takes place, according to Barrois, 

 by the separation of the internal mass of cells into mesoblast and hypoblast. 

 The proboscis is formed, according to this author, from the mesoblastic 

 tissues. Dieck, on the other hand, with greater probability, states that the 

 proboscis is formed by an invagination. In Cephalothrix a further point 

 deserves notice, in that the whole of the primitive epiblast becomes shed. 

 In this fact there may perhaps be recognised the last trace of a metamor- 

 phosis like that in the type of Desor. 



Delaminate types have been studied by Barrois (No. 192) and Hoffman 

 (No. 198), both of whom give circumstantial accounts of their develop- 

 ment. 



Hoffman's account is especially deserving of attention, since his observa- 

 tions were, to a great extent, made by means of artificial sections. The 

 following account is taken from him. His observations were made on 

 Tetrastemma varicolor, and Tetrastemma appears to be the genus in which 

 this type of development has been most completely made out. After a 

 regular segmentation the embryo forms a solid mass of cells, the outermost 

 of which soon become distinguished as a separate epiblastic layer. At the 

 same time the larva leaves the egg, and the epiblast cells become coated by 

 an uniform covering of cilia. At the anterior extremity of the body is a 

 bunch of long cilia ; and at the hinder end two stiff bristles are formed, but 

 soon disappear. 



The internal mass of cells is still quite uniform, but as the larva grows in 

 length the outermost of them arrange themselves as a columnar layer, 

 constituting the mesoblast. Of the cells internal to the mesoblast the outer 

 become columnar, and are converted into the walls of the alimentary tract, 

 while the inner ones undergo fatty degeneration, and form a kind of food- 

 yolk. In the later development the characters of the adult are gradually 

 acquired without metamorphosis, and the larval skin passes directly into 

 that of the adult. Both mouth and anus are formed nearly simultaneously 

 by a rupture of the enteric wall from within. The nervous system arises as 

 a thickening of the epiblast, which Hoffman states he has been able to see 

 in sections. Hoffman also states that the epithelium of the proboscis is 

 formed as a diverticulum of the alimentary tract, and that its sheath is 

 formed by a special mesoblastic growth. 



Barrois is less precise than Hoffman, from whom he differs in certain 

 particulars. Hoffman's statements about the proboscis are important if 

 accurate, but require further confirmation. 



Malacobdella. The early stages in development of the peculiar ecto- 

 parasitic Nemertine Malacobdella have been worked by Hoffman (No. 199) 

 by means of sections, and there appears to be a close agreement between 

 the development of Malacobdella and that of Tetrastemma. 



The segmentation is uniform, and there is no trace of a segmentation 



