LIFE HISTORY OF DESMOGXATHUS FUSCA. 267 



plete, the external gills fully developed, and the feet had the full 

 number of distinct toes (4 anteriorly and 5 posteriorly)." This 

 embryo was 13 mm. in length, 2 mm. shorter than the shortest 

 newly hatched larva which I have measured, and Wilder esti- 

 mated that its supply of yolk might indicate a continuation of 

 development within the egg for several days longer. Thus the 

 length of the entire period might be estimated at approximately 

 5 weeks. 



The length of the period of development within the egg varies, 

 however, even in the same batch of eggs, since the hatching proc- 

 ess sometimes continues for four or five days. This difference 

 in the time of hatching may possibly be due to the element 

 of chance in the occurrence of sufficient friction to rupture the 

 membranes; since, however, the newly hatched larvae have, so 

 far as my observations go, reached the same stage of development, 

 though differing slightly in size, it is more probable that this 

 difference in the date of hatching of members of the same brood 

 is due to an actual difference in the rate of development. The 

 difference which Wilder found in the segmentation stages of 

 eggs of the same brood, which might be logically accounted for 

 by a slight difference in the exact moment of fertilization, would 

 in itself be insufficient to account for so great a difference as 

 four or five days in the time of hatching, or for the decided 

 difference which I have observed in the development of embryos 

 taken simultaneously from the same batch of eggs during the 

 latter part of the embryonal period (Fig. 5, a and b). Piersol 

 ('09) noted a similar difference in the rate of development and 

 consequent time of hatching in the eggs of Plethodon cinereus 

 erythronotus , and attributed it to a difference in the accessibility 

 to a supply of oxygen, the eggs on the outside of the mass having 

 the better chance. In the case of Desmognathus, however, the 

 eggs are so arranged and so frequently shifted in relative position 

 by the movements of the mother, that it is hardly possible that 

 they differ greatly in their access to either moisture or oxygen 

 supply. Hilton noticed that the smaller eggs of the mass usually 

 develop more rapidly, a condition which might readily result 

 during the early stages from the smaller mass of inert yolk mate- 

 rial upon which to expend the energy of cell division, and possibly 



