544 LOCV. [Vol. XI. 



region, and there are also in this form ten or eleven segments 

 included in the expanded head part. In the chick there are 

 eleven segments in front of the first formed protovertebrae. 



9. Sinnmary. 



Some of the more important facts and conclusions regarding 

 the neural segment may be briefly stated : 



1. They are natural structures, not artifacts produced by 

 the reagents. This has been shown by their constancy in ap- 

 pearance and general characteristics when different reagents 

 are used; their consecutive history; their similarity in differ- 

 ent kinds of embryos; and lastly, by their presence in fresh 

 material before any reagents have been used. 



2. Early appearance. — The so-called neuromeric segmenta- 

 tion is the first to appear. It arises long before there are any 

 segmental divisions of the mesoderm, and therefore cannot 

 depend upon segmental divisions of the middle germ-layer. 

 Neuromeric segmentation is more primitive than mesodermic 

 segmentation. 



3. Structure. — The cells in these segments are character- 

 istically arranged even in the earliest stages, and their arrange- 

 ment and their structure would indicate that they are definite 

 differentiations of cell areas, not merely mechanical undula- 

 tions. 



4. Extent. — The entire embryo is divided into similar seg- 

 ments, and passes through an arthromeric condition similar to 

 that of arthropods and worms. In Squalus the neural segments 

 extend also into the germ-ring, and in the chick at times into 

 the primitive streak. 



5. Number. — There are in Squalus acanthias eleven seg- 

 ments in the brain region in front of the vagus nerve, and 

 fourteen paired segments in the entire brain region, as follows : 

 nine in the hind-brain, two in the mid-brain, and three in the 

 fore-brain; this does not include the anterior tip, which may 

 represent several consolidated segments. 



6. Backzvard grozvth. — There is some evidence to show that 

 the spinal cord region is being encroached upon by backward 



