POLYEMBRYONIC DEVELOPMENT IN TATUSIA 



657 



24 



26 



Fig. 22 A median section through plane a to 6, figure 1. The right and left 

 bays (It 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. X 130. 



Fig. 23 The section through e to / 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 (E. 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 A section across the tips of the secondary buds of Embryos III and IV. 

 X 153. 



