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of the lower hemisphere is so retarded by the yolk that the formation 

 of the grooves may be followed until the blastopore appears. In 

 order to trace the changes in both hemispheres at the same time 

 mirrors were placed beneath the eggs. 



Natural or artificial marks are frequently to be found on the sur- 

 face of the egg, and these often persist until long after the embryo 

 appears. By selecting such eggs any rotation or displacement which 

 occurs may be easily detected. Sometimes the surface niarkings are 

 obliterated during the pre-embryonic phases. In these cases a plane 

 passing through the primary ovic axis and the uppermost portion of 

 the blastopore has been considered as the median plane of the embryo. 

 Thirty eggs possessing such marks were selected from three nests, 

 containing an aggregate of one hundred seventeen eggs. These thirty 

 eggs were placed over mirrors, and diagrams of the successive cleavage 

 grooves and orienting marks made. Eight of these eggs died before 

 the median plane of the embryo was discernible, the high mortality 

 being due probably to the fact that the eggs were kept in a fixed 

 position. Twenty-two developed normal embryos, but for convenience 

 in arranging the figures only twenty are represented. Among these 

 there were but seven (1, 5, 6, 10, 15, 18, 19) in which the marks 

 aided in orienting the embryo. In the remainder the cleavage grooves 

 served as orienting lines until the dorsal lip of the blastopore was 

 formed, when the median plane of the embryo was determined by the 

 method described above. 



In the accompanying Figures 1—20, the first cleavage plane is 

 indicated by the heavy black line, while the median plane of the 

 embryo is represented by the arrow. A glance at these figures will 

 show that in two eggs (Figs, 1, 2) there is apparently coincidence 

 between the median plane of the embryo and the first cleavage plane. 

 The method not being absolutely exact, coincidence might have also 

 ocurred in the eggs represented by Figs. 3 and 11. An apparent co- 

 incidence between the second cleavage plane and the median plane of 

 the embryo was found in the egg shown in Fig. 10, and this might 

 also be the case in the egg depicted in Fig, 20. 



In the remaining eggs (Figs, 4, 5, 6, 7, 8, 9, 12, 13, 14, 15, 16, 

 17, 18, 19 j the angles formed by the median plane of the embryo and 

 the plane of either the first or second cleavage grooves, varied greatly. 

 It might be claimed that the plane of the embryo coincides with some 

 one of the four meridionals which constitute the third cleavage. Since 

 the observations were not made with reference to the planes formed 

 by the third cleavage grooves the writer is unable to offer evidence 



