CH. XIII] ORGANS FROM THE EXDODERM 139 



canal, the central canal of the nerve tube, is directly continued 

 into the archenteron (Fig. 37, A). At this time the archen- 

 teron is completely closed in from the exterior, since neither 

 the mouth nor the anus has as yet opened. 



The posterior ends of the medullary folds close just behind 

 the blastopore. The groove lying behind the blastopore is not 

 overarched by the folds. During this period the posterior pit 

 of this groove has become much deeper. At first, the pit was 

 separated from the archenteron by a thick layer of cells con- 

 sisting of ectoderm, mesoderm, and endoderm. The meso- 

 dermal cells begin to pull away from this region, and the pit, 

 in consequence, becomes deeper. Then the endodermal cells 

 pull away beneath the pit, and only a single layer of ecto- 

 clermal cells remains to separate the cavity of the archenteron 

 from the exterior. Finally the latter cells also draw away, 

 and the pit opens into the archenteron. The external opening 

 becomes the anus of the frog. It is at first almost on the 

 dorsal surface of the embryo, but it rapidly shifts 1 to a more 

 ventral position, and at the same time the region above it 

 elongates to form the beginning of the tail. The neurenteric 

 canal is only a temporary structure, and is soon obliterated by 

 the growing together of its walls, although its position may 

 be marked in sections for some time after its actual closure by 

 the irregular line of pigment in the region of the coalescence 

 of its walls. 



In the Urodela the changes that take place during the 

 final stages of the blastopore are somewhat simpler. The 

 circular blastopore is reduced to an elongated slit-like open- 

 ing ; but there seems to be some variation in the details of the 

 method of its later reduction. The medullary folds arch over 

 only the anterior end of the elongated blastopore, leaving free 

 the posterior end. The anterior end becomes the neurenteric 

 canal. The sides of the middle part of the slit-like blastopore 

 come together and fuse at the time of overgrowth of the med- 

 ullary folds. The posterior end of the blastopore always 

 remains open to the exterior, and forms the permanent anus. 



1 The method by which the apparent change in position of the anal opening 

 takes place has not "been clearly made out. 



