INTRODUCTION 461 



organ-forming area into a hollow tubular affair. With respect to the meso- 

 dermal areas, the immediate process is an internal splitting (delamination), 

 whereby the mesodermal area separates into an outer and an inner layer 

 with a space or cavity appearing between the two layers. In the case of the 

 teleost fishes, a process of internal separation of cells appears to play a part 

 also in the neural tubulation. 



b. Auxiliary Processes 



Aiding the above activities which produce tubulation are those procedures 

 which extend the tubulated areas into elongated structures. These auxihary 

 processes are as follows: 



( 1 ) The cephalic or head rudiment, with its contained fundaments of the 

 developing head region, grows forward as a distinct outgrowth. This 

 anterior protrusion is known as the cephalic or head outgrowth (figs. 

 223A, B; 232I-L). 



(2) The trunk rudiments enlarge and the trunk region as a whole under- 

 goes antero-posterior extension (figs. 225 A; 233). 



(3) The tail-bud area progresses caudally as the tail outgrowth and forms 

 the various rudimentary structures associated with the tail (figs. 225; 

 230F; 238). 



(4) A dorsal upgrowth (arching) movement occurs, most noticeable in 

 the trunk area. It serves to lift the dorsal or axial portion of the trunk 

 up above the yolk-laden area below, and the developing body tubes 

 and primitive body are projected dorsalward (figs. 221, 224, 241 ). 



(5) In embryos developing from rounded gastrulae, a ventral contraction 

 and reshaping of the entire ventro-lateral areas of the primitive trunk 

 region are effected as the yolk is used up in development. This results 

 in a gradual retraction of this area which eventually brings the ventro- 

 lateral region of the trunk into line with the growing head and tail 

 regions (cf. figs. 220, 223, 225 on the development of the frog, and 

 227 on the development of Necturus). 



(6) In embryos developing from flattened gastrulae, a constriction of the 

 ventral region of the developing trunk comes to pass. This constriction 

 is produced by an ingrowth toward the median line of entodermal, 

 mesodermal, and epidermal cellular layers in the form of folds, the 

 lateral body folds. Upon reaching the midline, the cellular layers fuse 

 as follows; The entodermal layer from one side fuses with the ento- 

 dermal layer of the other; the mesodermal layers fuse similarly; and, 

 finally, the epidermal layer from one side fuses with the epidermal 

 layer of the opposite side. The result is a general fusion of the re- 

 spective body layers from either side, as shown in figure 24 IC and D, 

 which establishes the ventral region of the trunk. A complete fusion 

 throughout the extent of the ventral body wall does not take place 



