POLYEMBRYONIC DEVELOPMENT IN TATUSIA 



601 



In the preserved condition, the chorionic vesicle measured 

 0.722 mm., from right to left, at the base, and the ectodermal 

 vesicle 0.443 mm. in the same plane; while in the antero-poste-- 

 rior plane it measured 0.866 mm., and the vesicle 0.312 mm. 

 The ectodermal vesicle is, therefore, approximately a third wider 

 in the right-left 'plane than in the antero-posterior plane, and 

 this difference is due to the presence of the primary buds. In 



Fig. 1 Outline reconstruction of specimen No. 247. This shows the well 

 formed 'primary buds,' situated on the right and left hand sides of the vesicle. 

 The broken line shows the plane of the two sections illustrated in plate 6, figures 

 60,61. X 95. 



preparing the specimen for microscopical study the sections were 

 cut so as to pass parallel to the long axis of the ectodermal vesi- 

 cle (fig. 1). 



In the median sections of the series the primary buds are in- 

 stantly recognized on the right and left sides of the ectodermal 

 vesicle. They appear as outgrowths from the vesicle, and their 

 cavities (fig. 22, R and L) are seen to be extensions of the com- 

 mon amniotic cavity. The wall of each bud is three or four 

 cells thick, while the roof of the vesicle has thinned out to a 

 single layer in thickness. Furthermore, the cells in the walls 



