44 



TEXT-BOOK OF EMBRYOLOGY. 



tral and peristomal mesoderm has already been stated, and since mesoderm 

 development is initiated shortly after the gastrula begins to elongate the 

 true gastral portion is relatively short. Whatever is added to this comes 

 from the region of the blastopore. In the germ ring cell proliferation con- 

 tinues rapidly and from the cells thus produced components of all three germ 

 layers are differentiated. In other words, the elongation of the embryo as a 

 whole, with its three germ layers, is due chiefly to this cell proliferation and 

 differentiation at its caudal end. Not only the mesoderm but also the gut, 

 the neural tube and other structures which will subsequently appear, in- 

 crease and develop from before backward. 



Anterior (cephalic) end 



Epidermis 

 (ectoderm) 



Entoderm 



Mesoderm 



Unsegmented 

 mesoderm 



Archenteron 



Posterior (caudal) end 

 FIG. 24. From horizontal section through Amphioxus embryo with 5 primitive segments; seen 



from dorsal side. Hatschek. 



The communication between the cavities of the primitive segments (ccelom) and the archenteron 



can be seen in the last 4 segments. 



The original gastral mesoderm gives rise to perhaps not more than the 

 first two pairs of somites. The succeeding somites arise from mesoderm 

 that originates in the region around the blastopore. By the time about 

 fourteen pairs of somites have developed the mesoderm no longer arises as 

 outgrowths from the entoderm of the gut wall but directly from the proliferat- 

 ing cells in the region around the neurenteric canal. As a matter of fact 

 the formation of somites now does not quite keep pace with the differentia- 

 tion of the middle layer and just in front of the blastoporal region there is a 

 short band of undivided mesoderm (Figs. 23 and 24). As this band grows 

 at its caudal end it is gradually being cut up into somites from its anterior 



