PBEFACE. 



GHEAT attention has of recent years been given to the study of 

 Embryology, and yet it is curiously difficult to find straight- 

 forward accounts of the development even of the commonest 

 animals. The special memoirs and monographs are usually 

 limited to particular phases in the life-history of the forms with 

 which they are concerned ; while the text-books of embryology 

 aim rather at explaining the general progress of development 

 within the several groups than at supplying complete descrip- 

 tions of individual examples. 



Up to the present time there has been no reasonably com- 

 plete account of the development of the common frog, or of 

 the rabbit, in our own or in any other language ; while in 

 works professing to deal with human embryology it is more 

 common than not to find that the descriptions, and the figures 

 given in illustration of them, are really taken, not from human 

 embryos at all, but from rabbits, pigs, chicken, or even 

 dogfish. 



This latter practice is a most unfortunate one, and has been 

 the cause of much confusion. The student is led to suppose 

 that our knowledge is more complete than is really the case, 

 while at the same time he finds the greatest difficulty in obtain- 

 ing definite information on any particular point in which he is 

 interested. Moreover, the implication that the details of develop- 

 ment are identical in members of the same or of allied groups is 

 directly opposed to the results of recent investigations, which 

 are showing more and more clearly that marked differences, 



