254 BULLETIN OF THE 



In this stage the segmental duct in the younger embryos shows some- 

 what different conditions from tliose found in the older embryos. 

 In the case of unhatched embryos possessing the simple pronephros 

 siiown in Figure 58, tlie duct on passing backwards gradually dimin- 

 ishes in calibre, and finally loses all trace of a lumen. The funda- 

 ment of the duct is in this region composed of four or five cells 

 in each cross section, which are frequently arranged with some regu- 

 larity about the centre as an axis. On proceeding to more posterior 

 regions the fundament of the duct becomes intimately connected with 

 the mesoderm, and is finally lost in that layer. In Amblystoma 

 the histological characters of the mesoderm and the ectoderm are 

 not sufficiently unlike to allow one to base on them a definite con- 

 clusion respecting the layer which has furnished the material for the 

 fundament of the duct. In all cases which I have observed, however, 

 the duct neither unites with the ectoderm nor terminates freely ; but 

 its posterior end invariably is closely applied to the mesoderm, and con- 

 sequently is most probably derived from that layer. In view of the fact 

 that the yolk spherules of the fundament of the duct are of the same 

 size as those present in the adjacent mesoderm, I am of opinion that 

 the duct has undergone no extensive independent growth, but has arisen 

 in situ as a proliferation of the somatopleure. 



In the older embryos of this stage, the duct has extended backwards 

 to the region of the cloaca, and joins the latter near the posterior face 

 of myotome XX. A distinct post-anal gut is present at this stage. Its 

 anterior portion contains an evident lumen, and appears as a direct con- 

 tinuation of the pre-anal portion ; its posterior tip is solid, and extends 

 backward into the tail region for the distance of about one millimeter. 

 From the ventral floor of this continuous intestinal tube, a median di- 

 verticulum leads backward and downward to the anus. The histological 

 characters of this diverticulum differ markedly from those of the rest of 

 the intestine, and by comparison with younger stages it becomes evi- 

 dent that the former has resulted from a proctodieal invagination. 

 Where the intestinal tube is joined by the proctodreura the ventral por- 

 tion, or cloaca, is T-shaped. The lateral arms receive the segmental 

 ducts, and the ventral stem may be followed to the anus. In Ambly- 

 stoma, then, the segmental ducts open into the intestine at the point 

 where the proctodseal ectoderm and the entoderm pass over into each 

 other. It is somewhat doubtful with which of the two germ layers the 

 wall of the ducts becomes continuous; but it is possible that — in con- 

 trast to the condition obtaining in the Anura studied — the ducts open 

 upon an ectodermal surface. 



