642 H. GRAHAM CANNON 
EXPLANATION OF PLATE 25. 
List of ABBREVIATIONS. 
bl, blastozone ; ect, ectoderm ; em, ectomesoderm ;_ end, endoderm ; 
ga, genital rudiment ; gac, cavity of genital rudiment ; 7, pit produced 
by invagination of genital rudiment ; me, mesendoderm ; mes, mesoderm ; 
mm, mesendodermal mass ; v.m, ventral mass; y.c, yolk-cells. 
Figs. 1, 2, 3, 4, 8, and 9, are from material fixed in Carnoy’s fluid. The 
remainder are from Gilson material. 
Fig. 1.—Section through an embryo showing the earliest sign of differen- 
tiation of the blastoderm. The ventral mass is marked off from the rest 
of the blastoderm as a group of cells completely pervaded by yolk. 
Fig. 2.—Median section through embryo showing differentiation of 
ventral mass into (1) genital rudiment, (2) anteriorly, the comparatively 
deeply staining mesendodermal mass, and (3) posteriorly, the ectomesoderm 
cells which are passing inwards. The nuclei at this stage are not at all 
distinct. 
Fig. 3.—Slightly oblique section—almost median—of an embryo slightly 
older than that figured in fig. 2. Shows the same as in fig. 2, but nuclei 
are now distinct. The cells surrounding the genital rudiment are seen to 
be pushing their way over the latter. 
Fig. 4.—Transverse section of the genital rudiment showing how the 
lips of the pit caused by its invagination are formed of inwardly migrating 
ectomesoderm cells. 
Fig. 5.—Transverse section through the genital rudiment after it has 
become completely internal, showing its cavity. 
Fig. 6.—Transverse section through the invaginating genital rudiment 
showing the fusion of the lips of the invagination pit. 
Fig. 7.—Median section showing commencement of mesendoderm. The 
genital rudiment is not yet completely internal. 
Fig. 8.—Median section showing mesendoderm growing backwards from 
the blastozone which is marked by a small depression. 
Fig. 9.—Median section. The mesendoderm has grown backwards 
underneath the genital rudiment which is now completely internal. 
Fig. 10.—Transverse section showing endoderm separated as a solid 
rod from the laterally lying mesoderm. The genital rudiment is imme- 
diately dorsal to the endoderm. Yolk-cells are seen in this figure enclosing 
yolk and oil-drops. 
Fig. 11.—Transverse section through the posterior region of the blasto- 
zone showing mesoderm formation at the lateral borders of the blastozone, 
