466 



DEVELOPMENT OF PRIMITIVE BODY FORM 



Fig. 223. Early neural tube stage of the frog, Rana pipiens, IV2 to 3 mm. in length. 

 (A) Dorsal view. (B) Midsagittal section of embryo similar to (A). (C) Same as 

 (B), showing organ-forming areas. Abbreviations: V. HD. = ventral hindgut divertic- 

 ulum; D. HD. = dorsal hindgut diverticulum; PHAR. = pharyngeal diverticulum of fore- 

 gut. (D) Later view of (A). (E) See fig. 224. 



between the keel method of the teleost and neural fold method of other verte- 

 brates described below (Selys-Longchamps, '10). 



2) Neural Fold Method. In the majority of vertebrates, the neural (medul- 

 lary) plate area folds inward (i.e., downward) to form a neural groove. This 

 neural groove formation is associated with an upward and median movement 

 of the epidermal layers, attached to the lateral margins of the neural plate, 

 as these margins fold inward to form the neural folds. A change of position in 

 the mesoderm also occurs at this time, for the upper part which forms the 

 somites shijts laterad from the notochordal area to a position between the 

 forming neural tube and the outside epidermis. This mesodermal migration 

 permits the neural tube to invaginate downward to contact the notochordal 

 area. Also, this change in position of the somitic mesoderm is a most important 

 factor in neuralization and neural tube development as mentioned at the end 

 of this chapter. (Note: In this stage of development, the embryo is often de- 



