The Process of Heredity 515 



For purposes of study lots of about fifty eggs were drawn off 

 periodically with a large pipette into Syracuse watch glasses and 

 examined with the low powers of the compound microscope. The 

 methods of study and observation were many and involved the 

 use of both living and preserved material. Comparisons of 

 differences between the four strains of each series were made in 

 various ways. In early stages actual counts of blastomeres were 

 made, both in living and in fixed material. In somewhat later 

 stages, when the blastodisc begins to spread out over the yolk, 

 diameter measurements of large numbers of blastodiscs were made 

 with the aid of the ocular micrometer. The latter instrument was 

 used for measuring head and body diameters, and other dimensions 

 of more advanced embryos. 



Camera lucida drawings of living individuals, selected as good 

 representatives of a strain, were made at selected intervals. These 

 camera drawings were made at the level of the table and showed 

 a magnification of 28 diameters. All drawings were made at 

 the same magnification. Details in some of the drawings were 

 filled in from fixed and stained material, put up at the time when 

 the drawings were made. 



The study of fixed material was of great assistance in confirm- 

 ing the observations made upon living material, and frequently 

 added much new data. 



Since no two series gave the same results, I have decided to 

 present the data of one successful series in the form of an abbrevi- 

 ated pictorial table (Plates II, III and IV). Other series can be 

 briefly compared or contrasted with this. Any one of five or six 

 series might equally well have been chosen, but this one is fairly 

 typical and shows perhaps a little more than any of the other 

 series examined. Some facts, however, are better brought out by 

 other series. No one series is complete for the reason that one 

 can make observations only at intervals and important stages 

 may be passed over between observations. The entire history, 

 however, can readily be pieced together from a considerable num- 

 ber of series. 



The four strains of the series selected are figured in tabular 

 form, nine stages being figured during the first week of develop- 



