192 EMBRYOLOGY 



envelope and as a means of making it of nearly the same 

 weight as water, thereby enabling it to float. Where cilia 

 are present, they are at first short, and only gradually in- 

 crease in length. In Bothriocephalus latus the exceedingly 

 delicate cilia attain a very great length. After the larva 

 has floated about in the water for a time, under certain con- 

 ditions for several days, it divests itself of the mantle, 

 whether ciliated or not. In many cases (as sometimes even 

 in Bothriocephalus latus) it may at the very beginning strip 

 off the mantle with the enveloping membrane. Even in this 

 naked condition the larva may live free for a time, but finally 

 perishes, if it finds no suitable host. 



ScHAUixsLAND explains the circumcrescence of the germ by the cap- 

 shaped cells, which occurs twice in nearly the same way, as an epiboly. 

 Accordingly he is compelled to assume a complete loss of the ectoderm in 

 the casting off of the superficial layer. The embryo is developed out of 

 the entoderm alone. He finds a support to this view in the fact that up 

 to the present time no actual body epithelium has been found either in 

 the Cestoda or in the Trematoda. This fact is in his opinion an argu- 

 ment that ectodermal structures are not present in these cases, a view 

 that Leuckart (No. 8) also maintains. In any event the origin of the 

 cuticula-like dermal layer merits a thorough investigation. If, as is to 

 be conjectured, it arises by the metamorphosis of a superficial cell-layer 

 (E. ZiEGLER, ScHWAEZE, ct alii), then it would correspond to the body epi- 

 thelium. The question whether in the casting off of the outer layer the 

 entire ectoderm is removed, or whether certain of its cells still remain 

 behind, must be difficult to determine on account of the small size of the 



^ [As is well known, a distinct epithelium could not be found on the 

 external surface in Cestodes and Trematodes. It was natural to connect 

 this fact, the absence of the body epithelium, with the casting off of the 

 external cell-layers in the embryo, and thus to assume that the entire 

 ectoderm was lost. A body epithelium, therefore, could not be present. 

 This question has often been considered, and even recently has been re- 

 sumed. While some investigators assume that the cuticula which covers 

 the body is secreted by the subcuticular layer, and that the latter is a 

 part of the body parenchyma (Bkaxdes, Loos), others maintain that it is 

 a metamorphosed epithelium, and believe they see, more or less distinctly, 

 cell nuclei retained in it (Braun, Monticelli). The most of these obser- 

 vations refer to the Trematodes, although investigations in this direction 

 have also been made on Cestodes (Zograff, Grassi ; see Appendix to 

 Literature on Cestoda). Zograff in particular finds that in various Ces- 



