ZOOLOGICAL SCIENCE 197 



correlate the attacks. Each supplements the other and sug- 

 gests new ways of advance. 11 



Although the field of embryology was among the earliest 

 to be invaded by the experimentalist in zoology, it is still 

 attractive, because there is no phenomenon of nature which 

 seems so inexplicable as the development of an adult indi- 

 vidual from a single cell. Unfortunately, many of the 

 organisms most desired for experimentation do not lend 

 themselves to particular experiments. The structure of 

 the animal and the nature of its environment impose limita- 

 tions. But the investigator's ingenuity frequently sur- 

 mounts difficulties which at first seem insurmountable. 

 What impresses those who worked as students during the 



11 During the last quarter of the nineteenth century, August Weismann 

 recognized the logical necessity of assuming germinal organization, in any 

 attempt to explain the physical basis of heredity. His book entitled "The 

 Germ-Plasm" (1893) postulated, theoretically, a germinal organization by 

 which the mechanism of heredity and development could be depicted. The 

 Weismannian doctrine fell into disrepute among biologists, because its author 

 set forth the organization of the germ-plasm upon a basis which seemed far too 

 theoretical. Biology was still under the spell of the epigenetic concept of 

 development, as established by von Baer during the first half of the nineteenth 

 century. The crude notions of preformation were clearly untenable and the 

 tendency was to regard the germ-plasm as undifferentiated protoplasm. Noth- 

 ing was known regarding Mendelian heredity with its implications regarding 

 the germ. Weismannism received wide discussion but scant acceptance. But 

 the work of the embryologists has since revealed a certain degree of organiza- 

 tion within the egg. Simple undifferentiated protoplasm has been found to be 

 non-existent, since all protoplasm is differentiated in some degree. Later, the 

 facts of Mendelian heredity have forced the postulation of a complex germinal 

 organization. To Weismann belongs the credit for recognizing the necessity of 

 assuming a germinal organization similar, in its causal relationship to the adult 

 organization, to that which the students of Mendelian heredity have postulated 

 on the factual basis of inheritance of unit-characters. The ridicule which 

 was for a time heaped upon Weismann's doctrine resembles that which at- 

 tended the theory of organic evolution for many years after it had been recog- 

 nized as a logical inference by the scientist-philosophers of the eighteenth 

 century. To-day experimental embryologist and geneticist virtually acknowl- 

 edge a Neo-Weismannism. The hypothetical germinal units of Weismann's 

 theory have been replaced by the hypothetical determiners or genes of the 

 Mendelian theory. It should be remembered that Weismann first elaborated 

 the theory of preformation in terms of cellular biology. 



