PROBLEMS OF MAMMALIAN DEVELOPMENT 455 



any great length of time, certainly not so long as from one preg- 

 nancy to the next. To believe in telegony it is necessary to sup- 

 pose either that the spermatozoa of a previous mating remain alive 

 in the female and fertilize some of her eggs in subsequent preg- 

 nancies, or that the development of a foetus from a given male in 

 some manner changes the hereditary complex that her germ cells 

 will transmit to subsequent generations. In the absence of any 

 scientific evidence that telegony exists, belief in its occurrence is 

 ridiculous, since it is opposed to the well-established facts of 

 heredity and reproduction. The alleged examples are to be 

 explained as the result of impurities in the stock of the female or 

 the male with which she is subsequently mated. The appearance 

 of such defects in the offspring of supposedly pure stock is easily 

 understood as the chance appearance of latent combinations 

 according to the laws of heredity. 



In concluding this general discussion of development, it may be 

 remarked that the field of Embryology is still attractive to investi- 

 gators because it includes so many unsolved problems. There 

 is, indeed, no phenomenon of nature which seems so inexplicable 

 as the development of a complex animal from a single cell, by cell 

 divisions and differentiations. The sequence of stages in develop- 

 ment has now been ascertained in a fairly complete manner for 

 representative types in each phylum of the Metazoa, as illustrated 

 by the accounts that have been given of individual animals. But 

 why the differentiation occurs at particular stages and in partic- 

 ular parts of the embryo is the great problem of Embryology to 

 which we are only beginning to find answers. The embryologist 

 is also confronted with the philosophical problems involved in the 

 origin of men from germ cells by a process of development. 



