64 WILLIAM PATTEN. 



Shortly after this stage the cerebral hemispheres become 

 more conspicuous. An outline of a section showing their 

 appearance at this time is seen in fig. 27. The whole brain 

 here appears to be a very thick, proliferating layer of ectoderm, 

 with no differentiation into an overlying hypodermis. The 

 underlying semicircular lobes are seen with their cavities 

 nearly obliterated (c. si.). 



Soon after this the ectoderm begins to advance in an 

 obscure fold over the cerebral hemispheres, as in fig. 28, which 

 represents a section just back of the posterior margin of the 

 semicircular lobes. In some cases one can see a double- 

 walled fold like that on the lower left side of the figure, but 

 usually there is only a single layer, the edge of which creeps 

 over the hemispheres, hugging closely to their outer surfaces. 

 Where this layer has passed, one sees a layer of thin cells {b. s.), 

 that probably develop into a part at least of the brain envelopes. 

 Whether this layer comes from the brain itself, the mesoderm, 

 or the middle wall of the ectodermic fold, could not be deter- 

 mined. 



Similar but more distinct medullary folds are seen advancing 

 over the margin of the mid-brain and ventral cords. Figs. 

 29 — 32 are selected from a series of cross-sections to illustrate 

 these folds. In fig. 29 the section passes just back of the 

 cerebral hemispheres in fig. 30, where the folds show best, 

 through the mid-brain, just in front of the cheliceree ; in 

 fig. 31 back of the second post-oral appendage; and in fig. 32 

 through about the middle of the third post-oral neuromere. 

 These sections show that the nervous system is not separated 

 from the surface by a process of delamination like that which 

 occurs in nearly all other Arthropods, but by an infolding 

 from the lateral margins, very similar to that which takes 

 place in Vertebrates, especially of such forms as the sturgeon, 

 as described by Salensky. The folds in the post-oral region 

 never advance far enough to meet over the median line. What 

 becomes of the middle layer of the fold when it does occur 

 could not be determined. Over the middle of the ventral 

 cords, mainly over the " Mittelstrang," there is formed by 



