PLATYELMINTHES. 199 



oesophagus (fig. 92 B, cs.}\ which soon fuse with the walls of the 

 body at the junction of the prostomial and metastomial plates 

 (fig. 92 C, .)> although they remain for some time attached to 

 the oesophagus by a solid cord. 



During these changes the original larval skin separates itself 

 from the subjacent layer formed by the discs (fig. 92, B and C), 

 and is soon thrown off completely, leaving the already ciliated 

 (fig. 92 C) external layer of the invaginated discs as the external 

 skin of the young Nemertine. During, and subsequently to, the 

 casting off of the embryonic skin, important changes take place 

 in the constitution of the various layers of the body, resulting in 

 the formation of the vascular system and other mesoblastic 

 organs, the nervous system, and the permanent alimentary tract. 

 These changes appear to me to stand in need of further elucida- 

 tion ; and the account below must be received with a certain 

 amount of caution. 



It has been already stated that the two discs give rise to fatty cells, 

 which occupy the space between the walls of the body and the archenteron. 

 At the period of the casting off of the embryonic skin fresh changes take 

 place. The discs become very much thickened, and then divide into two 

 layers, which become the epidermis and subjacent muscular layers. The 

 muscular layers arise in two masses, separated by the two cephalic sacks. 

 The anterior mass is formed as an unpaired anterior thickening, followed by 

 two lateral thickenings. The posterior mass is much thinner, in correspond- 

 ence with the rapid elongation of the metastomial portion of the embryo. 



The cells originally split off from the discs undergo considerable changes, 

 some of them arrange themselves around the proboscis as a definite mem- 

 brane, which becomes the proboscidean sheath^ some also form a true 

 splanchnic layer of mesoblast, and the remainder, which are especially con- 

 centrated during early embryonic life in the anterior parts of the body, form 

 the general interstitial connective tissue. The cephalic ganglia are stated to 

 become gradually differentiated in the prostomial mesoblast, and the two 

 cords connected with them in the metastomial mesoblast. 



At the time when the larval skin is cast off the original mouth becomes 

 closed, and it is not till some time afterwards that a permanent mouth is 

 formed in the same situation. During the early part of embryonic life the 

 intestine is lined with columnar cells, but, before the loss of the larval skin, 

 the walls of the intestine undergo a peculiar metamorphosis. Their cells 

 either fuse or become indistinguishable, and their protoplasm appears to 

 become converted into yolk-spherules, which fill up the whole space within 



1 Biitschli for Pilidium regards these pits as formed by invaginations of the epiblast, 

 but MetschnikofFs statements are in accordance with those in the text. 



