400 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



band (fi<r. IG, G), the edges of which come together beneath the band 

 and unite below it (H). The corresponding layers of the folds then 

 beconie continuous, and the embryo is now shut in beneath two cellu- 

 lar membranes (I). The outer membrane is called the serosa (Ser), 

 the inner, the amnion (.l?/i). The serosa commonly separates from 

 the entire surface of the yolk, forming a complete envelope about 

 the egg inside the vitelline membrane, the amnion covering usually 

 only the ventral part. 



There are many other variations in the details of the formation of 

 the embryonic coverings in the eggs of diiferent species of insects, 

 but since these protective membranes have nothing to do with the 

 true history of development, w^e need say no more about them here, 



Nbl 



Fig. 17. — The origin of tbe central nervous system 

 A, diagrammatic cross section of young embryo, showing ventral neural ridges 

 iNlR) and median neural groove (NIO) , with masses of primitive nerve cells, 

 or neuroblasts (Nbl) separated from the ectoderm (Ect) of the ridges and 

 ^iDove, I'orniinf,' twu lateral cords (LC), and a median cord (MV). 



15, Cross section through ventral part of young bee embryo, showing neuroblast 

 cells (Nbl) separating from cells of ectoderm (Ect) iu tbe neural ridgos and 

 groove. (Figure from Nelson.) 



C, cross section through older bee embryo, sliowing tlie ventral nerve trunk, 

 consisting at this stage of the lateral cords (LC) and median cord (MC) 

 entirely separated from the ectoderm (Ect) of body wall. (Figure from Nelson.) 



except to note that they are usually broken up, or are drawn back into 

 the yolk where they are absorbed, before the young insect hatches. 



THE NERVOUS SYSTEM 



It is most interesting to learn from embryologj' that the tissue of 

 the central nervous system, which in the adult is entirely inclosed 

 within the body, is derived from the ectoderm. Yet, if the develop- 

 ment of the individual follows the path of ancestral evolution, this 

 should not be surprising, because the outer surface of the animal, 

 being the part that comes into contact with the environment, should 

 be the part in which sensitive tissues would be most likely to develop. 



