104 ME. W. G. EIDEWOOB OK THE HYOBEANCHIAL 



from the hyoid by a distance equal to its own width; and the 

 breadth of the cartilage connecting the hyoid and branchial 

 divisions of the hyobranchial skeleton is nearly equal to the 

 antero-posterior diameter of the median basal plate, from which 

 latter the four great lobes, two hyoid and two branchial, radiate. 



The hyoid cornua are large, and each in shape resembles an 

 axe-head. The antero-external convex edge is thin and shows 

 signs of absorption. It is important to note that, although the 

 external or lateral extremities of the mandible and ceratohyal 

 are nearly in contact, a considerable space is enclosed between 

 their more median parts, and these relations are maintained as 

 long as the ceratohyal is recognizable. In Xenopus, it will be 

 remembered, the median ends of the ceratohyals lie close up 

 behind the mandible. Although the hyobranchial skeleton of 

 the early Pipa embryo exhibits the aforementioned peculiarities, 

 it is nevertheless far less aberrant than that of the Xenopus 

 larva. 



Parker states (33. p. 649) that there are neither external nor 

 internal gills in the embryos of Pipa, but it is evident that he had 

 never seen Wyman's paper describing (44. p. 371) the existence 

 of three pairs of external gills, internal gills, and a pair of 

 branchial clefts. It would be difficult to believe that a hyo- 

 branchial skeleton, conforming so closely to the normal anuran 

 type as does that of the present stage, should not be associated 

 with branchiae of some sort. 



Stage II. (PI. 11. fig. 5.) Emhryo with abdomen not distended 



hy yolk, altJiougli on dissection much is found still v/nabsorhed. 



Hind limbs flexed, 6 mon. in length ivhen straightened out. 



Fore limbs 3 mm. Length of body 12 mm., tail 11 mm. 



This stage does not differ to any great extent from the first, 



and is not so strictly intermediate between Stages I. and III. as 



could be wished ; but here again the deficiency is to be 



accounted for by the difG^culty of obtaining embryos of any 



particular age. The chief difference between this and the 



preceding stage is to be noted in the hyoid arch. Considerable 



absorption has taken place — a significant foreshadowing of the 



later complete disappearance of this arch. The erosion is 



greatest anteriorly, so that the antero-external edge, before 



Convex, is now concave, wliile the anterior indentation between 



the two cornua has broadened out. Since no absorption yet 



