616 



Chas. B. Wilson 



Accordingly the following experiments have been performed upon 

 different species simultaneously, and upon the same species at different 

 periods in its growth, with a view to determine, if possible, some 

 relation between changes in environment and rapidity of growth, and 

 more especially between such changes and the period of development 

 at which they occur. 



The eggs of the common spotted salamander (Amblystoma 

 punctatum), the large wood frog (Eana temporaria), and one of the 

 smaller wood frogs (Chorophilus triseriatus), were chosen because 

 they seemed particularly adapted for such experiments in two essential 

 particulars : 



They are quite common in the vicinity of Baltimore, and often 

 the three kinds can be found side by side in the same pool. The 

 second and more important essential is that they vary greatly in 

 the rapidity with which they develop. 



The Amblystoma eggs, as determined first by Clarke (8), and 

 later by Eycleshymer (14) and others, develop very slowly. 



According to Eycleshymer the rate of development is as 

 follows : 



Several hours intervene after fertilization before cleavage begins. 

 For this reason these eggs are more often found in an unsegmented 

 condition in the pools than those of the other two species. The 

 first cleavage occupies an hour and fifty minutes or more, the second 

 about an hour and forty minutes. From this point segmentation 

 progresses more rapidly, but unevenly, over the two hemispheres. 

 Gastrulation begins sixty hours after the first cleavage and the 

 blastopore closes about eighteen hours later. The embryo escapes 

 from the egg membranes in from eighteen to twenty-one days, and 

 reaches the adult condition in about one hundred days. 



The eggs of the large wood frog (Rana) develop more rapidly. 



Only a short time intervenes between fertilization and cleavage- 

 an hour or two. For this reason the eggs must usually be obtained 

 very early in the morning if they are to be found unsegmented. The 

 first cleavage occupies about an hour and the second somewhat less, 

 while the third and all subsequent cleavages occur at much shorter 

 intervals. Gastrulation begins toward the close of the second day, 

 and is completed eight or ten hours later. The embryo escapes from 

 the egg membranes during the latter half of the second week and 

 reaches the adult condition in about ten weeks (70 days). 



Being unable to find any account of the development of Choro- 



