164 NEMERTEA. 



metastomial fuse together, and finally the whole four unite into a 

 continuous ventral plate; analogous it would seem to the ventral 

 plate of chaetopodan and arthropodan embryos. The plate so formed 

 gradually extends itself so as to close over the dorsal surface, and to 

 form a complete skin within the primitive larval skin, which at this 

 period is richly ciliated, though the embryo is not yet hatched (fig. 

 91 C). While these changes are taking place, there are budded off 

 from the invaginated discs a number of fatty cells, which fill up the 

 space between the discs and the archenteron, and eventually form 

 the mesoblastic reticulum. During this stage the rudiment of the 

 proboscis also makes its appearance as a solid process of epiblast, 

 which grows backwards from the point of fusion of the two prostomial 

 discs at the front end of the embryo (fig. 91 C, pr.). A lumen 

 is excavated in it at a later period. The lateral organs or cephalic 

 pits arise in a somewhat unexpected fashion as a pair of diverticula 

 from the oesophagus (fig. 92 B, cs.) 1 , which soon fuse with the walls 

 of the body at the junction of the prostomial and metastomial plates 

 (fig. 92 C, cs.), 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 Cj, 

 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 elucidation; 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 ranch thickened, and then divided 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 cor- 

 respondence 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 



1 Biitsclilifor Pilidium regards these pits as formed by imaginations of the epiblast, 

 but Metschnikoff s statements are in accordance with those in the text. 



