THE SOMITES. 



29 



The cleavage of the mesoblast, however, does not extend as far as the mid-line 

 of the embryo, but ceases at some distance on either hand, thus leaving a tract of 

 uncleft mesoblast on either side of the medullary groove and the chorda. The 

 uncleft area constitutes "C^^ paraxial mesoblast (Fig. 32), which extends from the 

 head towards the caudal pole and appears upon the dorsal surface of the embryo as 

 two distinct ribbon-like tracts bordering the neural canal. Beyond the paraxial 

 mesoblast, the cleft portions of the middle layer extend on either side as the lateral 

 plates ; each lateral plate consists of two laminae, the one forming the dorsal and the 



Fig. 33. 



Fig. 34. 









Neural canal 

 Somite 



Chorda 



Intermediate cell- 

 mass 



Primary gut-tube 

 -Parietal mesoblast 

 -Visceral mesoblast 



Transverse section of human embryo of about 

 fifteen days, showing early differentiation of somite. 

 X 210. (Kollmann.) 



Neural canal 



Myotome 



Sclerotome 



Nephrotome 



Aorta 



Gut-tube 

 Coelom 



Transverse section of human embryo of about 

 twenty-one days, showing differentiation of somite. 

 X 90. i^KoUmann.) 



Muscle 

 plate 



Fig. 35. 



Dorsal border of 

 myotome 



Cutis-plate 



Other the ventral boundary of the enclosed primary body-cavity ; in view of their 

 subsequent relations to the formation of the body-walls and the digestive tube 

 respectively, the dorsal mesoblastic lamina is appropriately named the parietal layer 

 and the ventral lamina the visceral layer (Fig. 32). In the separation of these 

 layers, which soon takes place in consequence of the dorsal and ventral folding 

 occurring during the formation of the amnion and the gut-tube, the parietal mesoderm 

 adheres to the ectoblast, in conjunction with which 

 it constitutes the somatopleure (Fig. 29), the 

 ecto-mesoblastic sheet of great importance in the 

 production of the lateral and ventral body-walls. 

 Similarly, the visceral mesoblast unites with the 

 entoblast to form the splanchnopleure (Fig. 29), 

 the ento-mesoblastic layer from which the walls of 

 the primary digestive canal are formed. 



The Somites. — The paraxial mesoblast at an 

 early stage — about the twentieth day in man — exhibits 

 indications of transverse division, in consequence 

 of which this band-like area becomes differentiated 

 into a series of small quadrilateral masses, the 

 somites, or protovertebr(£. This segmentation of 

 the embryonic mass appears earliest at some 

 distance behind the cephalic end of the embryo, 

 at a point which later corresponds to the beginning 



of the cervical region. The somites are seen to best advantage in the human 

 embryo at about the twenty-eighth day (Fig. 71). 



The early somites, on transverse section, appear as irregular quadrilateral bodies, 

 composed of mesoblast and covered externally by ectoblast, lying on either side of 

 the neural canal and the notochord (Fig. 33). Each somite consists of a dorsomesial 

 principal cell-mass, which is connected with the lateral plate by means of an 

 intervening cell -aggregation, the intermediate cell-mass (Fig. 33). Subsequently, 



'^/%^ 



■^ISfe 



Sclero- 

 tome 





Differentiation of myotome of human 

 embryo of about twenty-one days. X 525. 

 {Kollmann.) 



