CONTRIBUTIONS TO THE ‘EMBRYOLOGY OF NEMERTEA. 447 
EXPLANATION OF PLATE XXII, 
Illustrating Professor A. A. W. Hubrecht’s “ Contributions to 
the Embryology of the Nemertea.” 
Diagrammatic sections through larve of Lineus at different stages. The 
primary epiblast is indicated by a light yellow, the secondary epiblast by a 
darker brown, the hypoblast by a pink, the mesoblast by a grey colour. Figs. 
1—6 and 14 and 15 are transverse sections, perpendicular to the longitudinal 
axis; Figs. 7—9 and 13 are longitudinal and parallel to this axis; Figs. 
10—12 horizontal. | 
Fic. 1.—The gastrula stage. 
Fic. 2.—First appearance of the mesoblast, and of the secondary epiblast. 
Fie. 3.—The secondary epiblast is gradually overcapped ; the dorsal portion 
of it arises by delamination. 
Fic. 4.—Two epiblastic invaginations (the cavities of the posterior 
brain-lobes or ‘cephalic sacs”) appear, right and left of the blastopore. 
Fic. 5.—The same invaginations after they have been nipped off; the 
secondary epiblast has increased; the mesoblast commences to accumulate 
against it; the blastopore is now slit-like. 
Fic. 6.—The incipient cavity of the posterior brain-lobes has shifted in 
position ; the mesoblast has acquired a more definite arrangement; in the 
hypoblast both portions are represented as covering each other; in the lower 
part of the anterior portion the incipient formation of the cesophageal 
epithelium is represented. 
Fic. 7.—Corresponds to a stage between that of Figs.2 and 3. First 
appearance of the proboscis. Subdivision of the archenteron. 
Fic. 8.—Corresponds to Fig. 5, but is taken parallel to but not coincident 
with the median plane. Thus the cephalic and ventral plates of the secondary 
epiblast are included in the section, and the incipient proboscis is not. 
Fic. 9.—Corresponds to Fig. 6. Further development of mesoblast round 
the proboscidian epithelium beneath the secondary epiblast and against the 
cesophagus, which is being formed out of the lower division of the wall of the 
anterior portion of the archenteron. The outer layer of the Desor larva 
ready to be stripped. 
Fie. 10.—Corresponds to Figs. 5 and 8. The proboscis epithelium is on 
the point of coalescing with the cephalic plates of the secondary epiblast. The 
flattened lumen of the anterior portion of the archenteron is indicated in this 
figure. 
