DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 383 
I. Changes in the Longitudinal and Transverse 
Diameters of the Ovum. 
Towards the end of the fifth day, or at the commencement 
of the sixth day, the longitudinal axis of the blastodermic 
vesicle is 125 uw long (fig. 5). During the sixth day that axis 
of the ovum is diminished, first to 95 mu (fig. 6), and then to 64 p, 
after which it again increases, and at the commencement of 
the seventh day it is 12ly. During the same period the 
transverse diameter increases from 26 » at the end of the fifth 
day to 64 u at the end of the sixth day. 
The first diminution in the length of the longitudinal axis 
is due to the invagination of the roof of the blastodermic 
vesicle into the cavity of the vesicle (fig. 6, Pl. XXIII); and 
were it not for this invagination the longitudinal axis of the 
ovum at the commencement of the sixth day would measure 
139 u (length of vesicle, fig. 6, Pl. XXIII, 95 w; length of mass 
of invagination cells, 44 1). 
The second diminution of the longitudinal axis results from 
the sinking of the invaginated mass of cells into the floor of 
the blastodermic cavity (fig. 7, Pl. XXIII). 
The increase in the longitudinal diameter towards the end 
of the sixth day is produced by the formation and distension 
of the cavity of the yolk-sac in the midst of the floor of the 
blastodermic vesicle (Y S, fig. 8, Pl. XXIII). 
The gradual increase of breadth is due partly to gradual 
multiplication of the cells of the ovum, and partly to the appear- 
ance and distension of the yolk-sac cavity. 
II. The Disappearance of the Primitive Cavity of 
the Blastoderm. 
The occlusion of this cavity is brought about by the in- 
vagination of the roof into its interior (fig. 7, Pl. XXIII), and 
by the growth of the yolk-sac (figs. 7 and 8, Pl. XXIII). 
