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. 



.k b 



I 



i 



J...LS 



■She./! £ik- ■ - 



Fig. 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 r61e it 

 plays is well illustrated in Figure 

 18, where (excepting the nerve 

 masses and the heart, which are 

 unrepresented) all of the differ- 

 entiated organs of the squid 

 which at this stage lie in the 

 sagittal plane are represented. 

 And these are seen to have 

 arisen by the foldings of the 

 unilaminar blastoderm which 

 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 

 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 



aB/ ttVo*?. 



Fig. 18. 



Fig. 17. Diagram representing the process of folding as it occurs in proto- 

 plasmic layers. By excessive growth between a and b, the fold a o b 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. & II., Fig. G73. 



