528 



BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



2) Contributions of the Hypomere (Lateral Plate Mesoderm) to the For- 

 mation of the Gut Tube and Heart Structures. Throughout the length of the 

 forming gut tube, from the oral area to the anal region, the lateral plate meso- 

 derm (mesoblast) contributes much to the forming gut tube. This is occa- 

 sioned to a great extent posterior to the pharyngeal area by the fact that the 

 inner or mesial walls of the two hypomeres enswathe the forming gut tube 

 as they fuse in the median plane (fig. 241), forming the dorsal and ventral 

 mesenteries of the gut. However, in the heart area, due to the dorsal dis- 

 placement of the foregut, the dorsal mesentery is vestigial or absent while 

 the ventral mesentery is increased in extent. Each mesial wall of the hypomeric 

 mesoderm, forming the ventral mesentery in the region of the developing 

 heart, becomes cupped around the primitive blood capillaries, coursing an- 

 teriad in this area to form the rudiments of the developing heart. The ventral 

 mesentery in the heart area thus gives origin to the dorsal mesocardium, the 

 ventral mesocardium, and the rudimentary, cup-shaped, epimyocardial struc- 

 tures around the fusing blood capillaries (figs. 236C-D; 254A). The primitive 

 blood capillaries soon unite to form the rudiment of the future endocardium 

 of the heart, while the enveloping epimyocardium establishes the rudiment of 

 the future muscle and connective tissues of the heart (Chap. 17). 



On the other hand, in the region of the stomach and continuing posteriorly 

 to the anal area of the gut, the movement mediad of the mesial walls of the 

 two lateral plate (hypomeric) mesodermal areas occurs in such a way as to 



ABDUCENS NERVE 



TH CRANIAL NERVE 

 AUDITORY CAPSULE 



3TH CRANIAL NER 



PROFUNDUS 

 (OPHTHALMICUS) 



DIVISION OF 

 TRIGEMINAL NERVE 



MANDIBULAR VISCERAL ARCH 



HYOID VISCERAL ARCH 

 BRANCHIAL POUCHES I'Sl 



SOMITES 1-8= THEORETICAL 

 SEGMENTS OF THE HEAD 



Fig. 253. Diagram illustrating the basic plan of the vertebrate head based upon the 

 shark, Scyllium canicula. (Modified from Goodrich: 1918, Quart. Jour. Micros. Sci- 

 ence, 63.) 



