No. J.J THE CELL-LINEAGE OF NEREIS. 425 



appendages (Fig. 92), A pair of frontal antennae {f.a) grow- 

 forth from the apical region, and a third pair of appendages (//.) 

 make their appearance just in front of the mouth. The latter 

 differ in form in the two species, being slender and pointed in 

 N. megalops, but blunt and rounded in N. limbata. These appen- 

 dages I take to be the " palpi." They, like the antennag and 

 cirrhi, bear a number of knobbed sense-hairs, some of which are 

 also found on the surface of the head itself. A pair of pigment- 

 spots also appear on the lower side of the head. 



On the upper side of the head, the supra-oesophageal or cere- 

 bral ganglia appear as an enormous bilobed thickening of the 

 ectoblast. They are distinctly posterior to the frontal antennae, 

 which extend straight forwards from the anterior extremity of 

 the body. Now, the "frontal bodies" are last seen in the 

 interval between the bases of the developing frontal anten- 

 nae ; i.e. they also are at the anterior extremity. In the spher- 

 ical trochophore, however (Figs. 84 to %6), they lie in front of 

 the cephalic nenral plate, and in front of the tipper pole. The 

 upper pole of the trochophore, therefore, does not coincide with tJie 

 ajiterior extreniity, but lies in the dorsal region. 



The history of the alimentary canal may be briefly dismissed. 

 As the body elongates, the stomodaeum changes its form, becom- 

 ing finally drawn out until it reaches the middle of the first 

 somite (Figs. 91, 92). The jaws appear within its cavity about 

 the fourth day. Meanwhile the mesenteron acquires a cavity, 

 which appears as a central space between the entomeres. I 

 need not describe the origin of the alimentary epithelium, as 

 this has been carefully studied by Salensky and v. Wisting- 

 hausen ; but I may call attention to the history of the oil-drops. 

 In the undivided Q.g% there are from ten to twenty of the large 

 oil-drops. As the segmentation proceeds, the drops in each 

 macromere gradually fuse, until only one drop is finally left 

 in each. The four oil-drops are arranged in two pairs, the 

 anterior of which, contained in the anterior entomeres {A and 

 B), are much smaller than the others. Since the drops do 

 not change their position with the movements of the embryo, 

 they form an admirable means of preliminary orientation, both 

 in living and in preserved specimens. The oil-drops persist 

 long after the appearance of the mesenteric cavity (Figs. 90, 91), 

 but are finally absorbed. The development of the proctodaeum 

 I have not yet fully investigated (see Salensky). 



