THE EARLY DEVELOPMENT OF THE FROG 111 



narrow just in front of the blastopore and widening gradually 

 as it extends up toward and beyond the upper pole. By the 

 time the yolk plug is withdrawn the margins of the medullary 

 plate are considerably thickened so that its outline is visible 

 externally; the median region has meanwhile become thinner 

 and a shallow groove results (Fig. 22, C). The thickened 

 margins are elevated slightly above the general surface of the 

 ectoderm and form the two lateral neural ridges, extending 

 from the sides of the blastopore along the dorso-lateral regions 

 of the embryo, widely separated, to a level about opposite the 

 blastopore, where they turn sharply and pass to the mid-line, 

 meeting and forming thus the transverse neural fold, which 

 marks the anterior limit of the medullary plate. The median 

 groove soon becomes quite pronounced and is known as the 

 neural groove. 



Sections through the region just in front of the blastopore 

 (Figs. 32, G; 33, F) show that the neural plate early begins to 

 separate from the remainder of the ectoderm. Fig. 44 shows 

 how the medullary plate is cut away laterally and superficially 

 from the ectoderm by a narrow split resulting from cell re- 

 arrangements; we have already seen that a similar space sepa- 

 rates the medullary plate from the underlying notochord. 



8. Summary and Comparisons with Other Forms 



Before continuing our account of the development of the 

 rudiments whose formation has just been described, we should 

 summarize the events of notogenesis and their relation to 

 gastrulation. It is evident that the distinction between gas- 

 trulation and notogenesis is real, and essential to an under- 

 standing of this period in development. Gastrulation involves 

 only those processes which convert the monodermic embryo 

 into a clidermic embryo; notogenesis includes those processes 

 involved in the formation of the medullary plate, notochord, 

 and mesoderm. Gastrulation is accomplished primarily by de- 

 lamination and the rearrangement of the yolk cells, and only 

 secondarily, and to a very slight extent, by invagination. In 



