428 MALL. [Vor. XII. 
XII, and the ventral walls of the pericardial space are composed 
wholly of mesoderm. This indicates that the growth of this 
wall was first by a union of the mesoderm, which was followed 
by the ectoderm of the amnion to complete the body wall. 
The process is shown in Figs. 22-24. Fig. 22 is a hypo- 
thetical stage between Graf Spee’s embryo Gle. and my embryo 
No. III. As the process from Graf Spee’s embryo continues, 
the blood-vessels reach the body to form the heart, as indicated 
by the outlines marked v. in Fig. 22. The mesoderm of the 
amnion then unites with that of the um- 
bilical vesicle, and the first pericardial 
space is formed. This is not wholly an 
imaginary stage, for it is based upon 
Bonnet’s observations upon the sheep,! 
as well as Cadiat’s upon the chick.? In 
a sagittal section of a sheep’s embryo 
of about the same stage (Plate III, Figs. 
16-20, c CB) Bonnet gives a similar fold, 
and after the pericardial walls are well 
BiG, 25 go coca tircughthe Hee eTOrmed he sives.an iivistration ol a,staeic 
of Embryo No. III. Enlarged 
ss times. P%, pharynx; H,heart. 1n Which it still communicates with the 
he arn i the amsotis cavity extraembryonic coelom (Plate IV, Fig. 
ure growth of the amnion tocom- 77, KC), With Graf Spee’s embryo 
plete the ventral body wall. ; j 
Gle. and with Bonnet’s observations 
upon the sheep as a starting-point, it is not difficult to interpret 
Figs. 22-24. 
Extension of the Amnion. — After the stage of embryo XII 
is passed the amnion rapidly envelops the whole body and soon 
passes out over the cord. The next stage after No. XII which 
I have studied is No. XIX. I have very perfect photographs 
of this specimen, and the sections are all good, although the 
nervous system is macerated. The embryo has rotated in the 
amnion, throwing the cord to the right side with the left side 
towards the observer. It would have been impossible to obtain 
a view of the right side of the embryo without cutting the 
cord. The outlines of this embryo and ovum are given in 

1 Bonnet: His’s Archiv, 1889. 
2 Cadiat: Jour. de l’Anat. et de la Physiol., 1883, Plate V, Figs. 1, 2. 
