V-] 



THE TAIL-FOLD. 



125 



the whole archiblastic framework of the embryo became riddled by a network 

 of parablast. From these parablastic elements he considered there arose not 

 only the epithelium (endothelium) of the blood-vessels and lymphatic spaces, 

 but also all the connective-tissue elements of the body, the archiblast being 

 represented in the vessels of the adult body by the muscular fibres alone. 



17. As we stated above (p. 87), the folding in of the 

 splanchnopleure to form the alimentary canal is proceeding 

 with great rapidity, the tail-fold as well as the head -fold con- 

 tributing largely to the result. 



The formation of the tail-fold is very similar to that of 

 the head-fold. At the extreme hind end of the embryo, at 

 the tip of the tail (Fig. 40, i), there is no cleavage of the 

 mesoblast, and therefore neither somatopleure nor splanchno- 

 pleure. The tail is a solid, somewhat curved, blunt cone of 

 mesoblast, immediately coated with the superficial epiblast 



FIG. 40. 



SECTION or THE TAIL-END OF AN EMBRYO (CHICK) OF THE THIRD DAT. (From 



Dobrynin. ) 



t. the tail. So. somatopleure, Spl. splanchnopleure. pp. pleuroperitoneal space. 

 The letters G and Dd are placed within the alimentary canal : a more 

 detailed explanation of the figure is given under Fig. 50. 



except at the upper surface (corresponding to the back of the 

 embryo), where lies the pointed termination of the neural 

 tube. At some little distance forwards, towards the head, 

 the cleavage of the mesoblast begins, and the somatopleure 

 diverges from the splanchnopleure, the latter, as in the head- 

 fold, being folded in to a greater extent than the former. Ex- 

 cept for the absence of the cephalic enlargement of the neural 

 tube, the presence of distinct protovertebrae up to nearly the 



