503 The Embryology of Hylodes Martini censis 



Resemblances. 



1. Holoblastic segmentation. 



2. The mode of formation of the blastopore, the presence of the yolk 

 plug, and its later withdrawal. 



3. The mode of development of the liver, pancreas and lungs. 



4. Development of the pronephros (with its three funnels) of the 

 Wolffian duct, and of the mesonephros. 



5. The presence of a tail. 



Differejices. 



1. A solid medullary plate. 

 3. In the alimentary canal : 



(a) Absence of horny jaws and teeth. 



(b) Probable absence of perforated gill-slits. 



(c) Absence of external and internal gills. 



(d) Absence of coiled intestine. 



(e) Extensive closure of the middle region of the archenteron. 



3. Simplification of the circulation in the gill region. 



4. Early and simultaneous appearance of all four legs. 



5. Incomplete development of the lungs and of the pulmonary circu- 

 lation at the time of hatching as compared with the tadpole when it 

 leaves the water. 



6. Forelegs are never completely enclosed by an operculum. 



7. Changes in the tail from an organ of locomotion to one of res- 

 piration. 



8. Presence of a hardened tip on the upper lip. 



9. Absence of adhesive glands. 



On the assumption that Hylodes has had an ancestor in which a tad- 

 pole stage existed, the development has been altered by losses, by addi- 

 tions, and by changes in organs already present. The structures that 

 have been lost are : the horny jaws and teeth, adhesive glands, the perfor- 

 ated gill-slits, the external and internal gills, the coiled intestine, and 

 certain blood-vessels of the gill region and of the lungs. There have been 

 added the hard tip of the upper lip, and the early development of the legs. 

 The tail has changed from a muscular to a vascular organ. 



Gekmantown, Pa., U. S. A., February, 1904. 



