ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 203 



hand a tubular sheath, which is continuous at one place by means 

 of a connecting cord with the flattened remaining portion of the 

 cloth. 



A process similar to the externally visible one just described, by 

 which the lateral and ventral walls of the body are produced from 

 the sheet-like fundaments, takes place at the same time within the 

 embryo in the splanchnopleure. There are developed from it, as 

 from the somatopleure, an anterior, a posterior, and two lateral 

 intestinal folds. 



First, at the time when the head is differentiated (fig. 122), the 

 part of the splanchnopleure corresponding to it (F.Sp.) is folded 

 together into a tube, the so-called cavity of the fore gut or head-gut (D). 



The same process repeats itself on the third day of incubation at 

 the posterior end of the embryonal fundament, where, upon the 

 appearance of the caudal part (Plate I., fig. 11), there is formed 

 within it and out of the splanchnopleure the cavity of the hind gut. 



Both parts of the intestine at first terminate with blind ends 

 directed toward the outer surface of the body. At the head-end 

 the mouth-opening is still wanting, at the posterior end the anus. 

 When, however, one raises the blastoderm with the nascent embryo 

 from the yolk, and examines it from the under side, the anterior 

 and posterior portions of the intestinal canal exhibit openings (vdpf 

 and hdpf}. through which one can look from the yolk-side into the 

 blind-ending cavities. One of these is called the anterior, the other 

 the posterior, intestinal portal or intestinal entrance (Plate I., fig. 11 

 vdpf and hdpf). 



Between the two portals the middle region of the intestinal canal 

 remains for a long time as a leaf-like fundament. Then by its 

 becoming somewhat bent downwards (Plate I., figs. 9 and 2) there 

 arises under the chorda dorsalis an intestinal groove (dr), which lies 

 between fore and hind gut. Owing to the further increase of the 

 lateral intestinal folds (df), the groove becomes deeper and deeper, 

 and finally, by the approximation of the edges of the folds from in 

 front, from behind, and from both sides, becomes closed into a tube 

 in the same manner as the wall of the body. 



At only one small place, which is indicated by the ring-like line 

 dn in Plate I., figs. 3 and 10, the folding and constricting-off process 

 is not completed, and here the intestinal tube too remains con- 

 tinuous, by means of a hollow stalk, with the extra-embryonic part 

 of the splanchnopleure, which encloses the yolk. 



The part of the germ-layers which is not employed in the formation 



