SKELETON AND LAEYNX OF XENOPFS AND PIPA. 103 



Fipa are less dependent on external conditions, being provided 

 at the outset with a large quantity of food-yolk, and being 

 lodged during their development in the integumentary pits on 

 the back of the mother. But it happens unfortunately that very 

 little idea of the extent of development of the hyobranchial 

 skeleton can be obtained from an examination of the external 

 features, and it is consequently necessary in most cases to 

 dissect out this portion of its body before deciding what stage 

 any particular embryo has reached. The table on p. 102, com- 

 piled from the Pipa embryos examined, will show how useless 

 actual measurements are for discriminating the various stages. 

 Stage I. is distinguished by the fact that the fore limbs are not 

 yet extruded, and Stage VI. is marked by the complete absorp- 

 tion of the tail ; but there is no safe guide for distinguishing the 

 intermediate stages. 



Stage I. (PI. 11. fig. 4.) Mnibryo with abdomen much distended 

 ly unabsorhed yolk. Fore limbs not yet extruded. Length 

 of body 10 mm., tail 12 mm. 



I have called this Stage I. because it is the youngest that I 

 have been able to examine ; it is, however, highly probable that 

 the hyobranchial skeleton of younger embryos would show many 

 features of interest. Owing to the fact that the larvae of 

 Xenopus are freely-swimming tadpoles, while the young of Plpa 

 only quit the integumentary pits on the back of the mother after 

 their metamorphosis is complete, it is no easy matter to correlate 

 the stages of development in the two genera ; but, judging from the 

 extent of development of the hyobranchial skeleton alone, this 

 first stage of Pipa would seem to be equivalent to a stage inter- 

 mediate between those numbered II. and III. in Xenopus. The 

 branchial arches are rods of cartilage and not sheets, as in the 

 early stages of Xenopus, and the lines of demarcation of the five 

 elements of the larval hyobranchial skeleton are not to be 

 distinguished. The hyobranchial skeleton is one continuous 

 cartilage, considerably broader across the branchial than across 

 the hyoid region. The branchial skeleton has the appearance of 

 having already suffered considerable absorption. The larynx lies 

 in the deep indentation in the middle of the posterior border, and 

 does not overlie the branchial cartilage. The three branchial 

 clefts are approximately equal in length, but in width the pos- 

 terior has the advantage. The first branchial arch is separated 



