186 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
in area, the latter is compelled to fold, producing, according to the point 
of reference, an elevation above or a depression below the general level. 
óeÓÓ--—--2---- - 
Fre. 17. 
So important relatively is the process of folding that it has apparently 
been regarded by some authors as almost the sole developmental pro- 
cess, How important a róle it 
plays is well illustrated in Figure 
18, where (excepting the nerve 
Shell $! masses and the heart, which are 
PECA unrepresented) all of the differ- 
ITnargin. . . 
Pisis qui entiated organs of the squid 
ritate Zunnen Which at this stage lie in the 
e Fiona 
sagittal plane are represented. 
=-Otocyet. 
And these are seen to have 
Radurm-N arisen by the foldings of the 
fae. E . . . 
EN unilaminar blastoderm which 
Aland, 4 
a5 covers the yolk. The connec- 
tions of the otocyst and the 
shell-gland with the exterior 
are now lost, but fin, mantle 
margins, funnel, and the various 
Fic. 18. appendages of the alimentary 
tract, as well as the tract itself, 
are all seen to be arising as foldings of the blastoderm. This view does, 
Dig. Diagram representing the process of folding as it oceurs in proto- 
plasmic layers. By excessive growth between a and b, the fold aob is pro- 
duced. 
Fig. 18. Sagittal section of an embryo of Loligo vulgaris, showing the origin of 
organs through foldings of the blastoderm. From K. € IL, Fig. 073. 
