518 H. H. Nt'iuman 



ments of the blastodiscs of twenty eggs of each of the four strains 

 gave the following figures : 



H pure (Hi): 16, 18, 16, 17, 17, 15, 18, 18, 16, 17, 17, 16, 



15, 18, 18, 18, 17, 18, 17, 17. Average 17+ mm. 

 H hybrid (hi): 13, 13, 15, 14, 14, 15, 11, 17, 12, 16, 14, 



13, 18, 12, 12, 13, 14, 15, 16, 14. Average 14 mm. 

 M hybrid (mi): 14, 14, 16, 17, 12, I2, 13, 18, 18, 17, 12, 



II, 14, 14, 14, 15, 15, 16, 18, 18. Average 15— mm. 

 M pure (Mi): 15, 16, 15, 15, 16, 17, 14, 15, 15, 15, 15, 15, 



15, 16, 15, 16, 16, 14, 13, 14. Average 15 + mm. 



These measurements were made in less than half an hour 

 in the order listed. The H hybrids had about fifteen minutes 

 advantage in time over the H pures and yet show a lower average 

 blastodisc diameter. A higher degree of variability is also notice- 

 able at this early period. The measurements of the two majalis 

 strains show no marked differences between them. The slightly 

 greater average diameter of the M pures might be due to the 

 time advantage of about fifteen minutes that elapsed between the 

 measurements of the two strains. After determining the average 

 diameters of blastodiscs in all four strains camera drawings 

 of an average embryo of each strain were made and are reproduced 

 in the table. The measurements made with the ocular microm- 

 eter, when compared with a micrometer scale on the stage, 

 showed a magnification of 17 diameters, so that, in order to reduce 

 the measurements to actual millimeters the figures must be divided 



by 17- 



The diff^erence in the average diameters of blastodiscs between 

 H pure and H hybrid was so marked that it must have been 

 evident for some time. 



Stage 2 (24 hours) (Plate II): The majority of the H pure 

 embryos showed the germ ring nearly halfway around the yolk 

 and a fairly well defined embryonic shield. The condition seen in 

 practically all of the specimens of H hybrids was distinctly less 

 advanced than in the H pures, the blastodisc still forming a 

 shallow cap over the yolk mass. No measurements were made of 



