POLYEMBRYONIC DEVELOPMENT IN TATUSIA 605 



bryos. The 'right-lateral' (II) and 'left-lateral' (IV) embryos 

 lie at the acute angles of the diamond, while the 'dorsal' (III) 

 and 'ventral' (I) embryos occupy the obtuse angles. The same 

 condition prevails in two specimens but slightly older than this 

 one; hence we may conclude that it is the normal arrangement 

 of the secondary buds. 



It is certain from the evidence that Embryos I and II are the 

 product of the right-hand primary bud, while Embryos III and 

 IV have come from the left-hand primary bud. It might appear 

 from an examination of figure 2 that the secondary buds at II 

 and IV had alone come from the two primary buds of such a 

 specimen as No. 247 (fig. 1), while those at III and I had 

 arisen de novo from the dorsal and ventral sides, respectively, 

 of the ectodermal vesicle. But the entire genesis of these sec- 

 ondary buds argues against such an interpretation, as will be- 

 come clear after a complete account of their development has 

 been given. Nevertheless, it 'is an interesting fact, and one I 

 believe not to be without significance, that the secondary buds 

 II and IV occupy positions on the periphery similar to those of 

 the two primary buds. Furthermore, it is probably correct to 

 regard Embryos I and III as having arisen as outgrowths from 

 the left sides (assuming that the observer is situated hi the cen- 

 ter of the free surface of the blastocyst) of the right and left 

 primary buds, respectively. 



The oval figure in each bud indicates the area over which the 

 ectoderm and entoderm have come into intimate contact or have 

 fused, while the solid line within each oval area shows the posi- 

 tion and extent of the primitive groove. The head of the future 

 embryo will, in each case, be directed towards the center of the 

 vesicle. 



The plane of the sections is indicated by the broken parallel 

 lines, which also show the position of the four sections from 

 which the photographs of plate 7 were made. 



The section passing through the plane e to f will be taken for 

 detailed account of the different parts of the chorionic vesicle. 

 The outer membrane of the chorionic ectoderm (fig. 23) shows 

 signs of breaking away. It has usually disappeared before this 



