

PREFACE 



THE development of the frog's egg was first made known 

 through the studies of Swammerdam, Spallanzani, Rusconi, 

 and von Baer. Their work laid the basis for all later research. 

 More recently the experiments of Pfliiger and of Roux on this 

 egg have turned the attention of embryologists to the study 

 of development from an experimental standpoint. Owing to 

 the ease with which the frog's egg can be obtained, and its 

 tenacity of life in a confined space, as well as its suitability for 

 experimental work, it is an admirable subject with which to 

 begin the study of vertebrate development. 



In the following pages an attempt is made to bring together 

 the most important results of studies of the development of 

 the frog's egg. I have attempted to give a continuous account 

 of the development, as far as that is possible, from the time 

 when the egg is forming to the moment when the young tad- 

 pole issues from the jelly-membranes. Especial weight has 

 been laid on the results of experimental work, in the belief 

 that the evidence from this source is the most instructive for 

 an interpretation of the development. The evidence from the 

 study of the normal development has, however, not been neg- 

 lected, and wherever it has been possible I have attempted to 

 combine the results of experiment and of observation, with the 

 hope of more fully elucidating the changes that take place. 

 Occasionally departures have been made from the immediate 

 subject in hand in order to consider the results of other work 

 having a close bearing on the problem under discussion. I 

 have done this in the hope of pointing out more definite con- 

 clusions than could be drawn from the evidence of the frog's 

 egg alone. 



In treating the general problems of development, I have tried 

 to keep as near to the evidence as possible. I have intention- 



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