THE EARLY DEVELOPMENT OF THE FROG 123 



the margin of the nervous system. The mandibular arches are 

 less marked; these appeared even before the medullary plate 

 became folded together, as a pair of low ridges diverging from 

 the antero-lateral regions of the plate and sometimes called 

 the " sense plates." They lie between the olfactory and optic 

 rudiments and form the antero-lateral regions of the embryonic 

 head. 



4. The Mesoderm 



The delamination of the mesoderm from the surface of the 

 endoderm commenced in the dorso-lateral regions anteriorly, 

 and spread thence posteriorly and ventrally. We have seen 

 that posteriorly the mesoderm finally passes into the region of 

 the germ ring, or what now represents a portion of that, the 

 primitive streak, where it continues to be formed and budded 

 off anteriorly as the primitive streak extends posteriorly. And 

 ventrally the delamination ceased in the ventro-lateral regions 

 of the endoderm, and the mesoderm then extended gradually 

 toward the ventral side through the multiplication of its own 

 cells and their downward extension, and through the splitting 

 off of scattered groups of cells from the endoderm toward the 

 ventral side. In this early embryo the mesoderm forms a 

 distinct layer separating ectoderm and endoderm throughout 

 (Fig. 44), except in the primitive streak and in the head region, 

 where the mesoderm is never in the form of a definite sheet, but 

 is represented by scattered cells filling the spaces between the 

 wall of the mesenteron and the nervous system and ectoderm 

 (Fig. 45). In the pharyngeal region the mesoderm becomes 

 interrupted by the extension of the gill pouches out to the ecto- 

 derm; between successive pouches groups of mesoderm cells 

 are enclosed which become the rudiments of the visceral arches 

 mandibular, hyoid, and branchial. 



Through the trunk region the typical mesodermal structures 

 develop rapidly. The axial region toward the chorda thickens 

 and becomes solid; this is the segmental plate or myotomal region 

 (Fig. 39) . Farther laterally the thinner layer is the lateral plate; 



