THE PROBLEM 39 



cognizable by their structure, and often are set aside 

 in a definite location in the developing embryo. Thus, 

 to take but a single example of a phenomenon of wide 

 generality, at a very early stage in the development of 

 the dog-fish, when the only bodily organs of which even 

 the rudiments are recognizable are the beginnings of what 

 will presently become the spinal cord and the back-bone, 



_ FIG. 10. First division in egg of Cyclops, showing at one pole of spindle the granules 

 which mark the germ path. (From Child, after Amma, by permission of University of 



Chicago Press). 



it was shown by Woods, many years ago, that the germ 

 cells are definitely localized and recognizable, as shown 

 in Figure 9. 



In some forms, notably the round-worm Ascaris, va- 

 rious Crustacea and insects, the cells which are to become 

 germ cells are visibly set apart from the very first or one 

 of the first three or four cleavages of the fertilized ovum. 

 For example, in the case of the crustacean Cyclops, Amma 

 has shown that the granules visible at one pole in the 

 very first division mark the prospective germ path, as 

 shown in Figure 10. 



In the gnat Chironomus the same thing is visible at a 

 very early cleavage, according to the observations of 

 Harper. For a comprehensive and critical review of the 



