POLYEMBRYONIC DEVELOPMENT IN TATUSIA 657 
Fig. 22 A median section through plane a to b, figure 1. The right and left 
bays (? and L) of the primary buds are well shown in the figure. A remnant of 
the partition which previously separated the two mesodermal vesicles is seen at 
par. >< 130! 
Fig. 23 The section through e to f of figure 2. It shows the transverse section 
across Embryos I and II. This specimen shows a rather rare condition for a 
blastocyst so far advanced in development, in that the chorionic ectoderm has not 
as yet disappeared. However, it shows many signs of degenerating. X 71. 
Fig. 24 Enlarged view of the embryonic portion of the ectodermal vesicle of 
the preceding figure. The figure is inverted in order to bring the dorsal sides of 
the embryonic primordia uppermost. The primitive streak region of each embryo 
is proliferating mesodermal cells (#. Mes.). In the case of Embryo I the section 
cuts across a pit-like depression (P.P.) of the primitive groove. X 153. 
Fig. 25 A portion of the section passing through the anterior end of Embryo 
II. It shows the entoderm beginning to thicken preparatory to a proliferation of 
mesodermal cells. This region of the entoderm corresponds to the protochordal 
plate of Hubrecht. X 153. 
Fig. 26 Asection across the tips of the secondary buds of Embryos III and IV. 
x 153. 
