A414 MALL. iVor. Xi. 
The ectodermic plate in the large invagination of the amnion 
is very broad, but not of equal thickness throughout its extent, 
and it ends very abruptly beyond the opening upon the surface. 
As the opening approaches the stem, the cells of the ectoderm 
are continued somewhat along its surface, as indicated by the 
black line in Fig. 8. 
All of the space between the embryonic vesicle and the 
chorion is the coelom, and in this specimen it communicates 
with the amnion. Whether this is transient or unusual can- 
not, of course, be stated. Should further experience show that 
the amnion is closed at an earlier stage than indicated in this 
specimen, it would not materially affect my diagrams or obser- 
vations. Graf Spee’s recent observation, Fig. 14, makes him 
think that this is the case, but it is just as easy to interpret 
the formation of the amnion in Fig. 14 from that in Fig. 8 as 
by his theory. 
The next stages in the development of the embryonic vesicle 
are taken from Graf Spee, and they are of importance to eluci- 
date the changes which take place preparatory to the forma- 
tion of the body cavity. In Fig. 14, which represents the 
younger embryo, the amnion is still surrounded completely 
with mesoderm, as in embryo No. XI, represented in Fig. 8. 
The mesoderm crosses the median line, as the sections given 
by Graf Spee! show. The dorsal side of the amnion is cov- 
ered with a very thick layer of mesoderm, as the closure of the 
amnion in embryo No. XI would suggest. 
From the stage represented in Fig. 14 it is easy to pass to 
the older embryo represented in Fig. 15. Now the body of 
the embryo is well marked, the neural folds are just beginning, 
and the neurenteric canal has just been formed. The chorda 
dorsalis is not yet separated from the entoderm, and the blood 
islands encircle completely the umbilical vesicle and have nearly 
reached the head end of the body of the embryo preparatory to 
the formation of the heart. 
It is not very difficult to imagine the embryonic vesicle of 
Fig. 8 to be converted into the vesicle of Fig. 14. To be sure, 
the invagination in Fig. 8 seems to be much larger than neces- 
1 Von Spee: His’s Archiv, 1896, Plate I, Figs. 4, 5, 9, and Io, 
