THE PROTOVERTEBRAL SOMITES. 25 
are formed, the protovertebral somites are connected with the lateral mesoderm by 
somewhat contracted strands of cells which collectively constitute the intermediate 
cell mass of each side (Fig. 14). 
The separation of the paraxial mesoderm from the lateral plates and the 
segmentation of the former into somites extends forwards to the region of the hind 
brain, where the first protovertebral somite is formed. In front of this the 
‘mesoderm, in mammals at least, does not become segmented. 
The cavity of the cclom may extend into the paraxial mesoderm before it is 
segmented into protovertebral somites, or it may stop just outside the limits of the 
paraxial mesoderm. In the former case each somite, when separated from the 
lateral plate, contains a cavity, and the intermediate cell mass is also hollow for a 
I PA 
EC 
SoM SoM 
Fic. 19. —EarbLy STaGEs IN THE FoLDING OFF OF THE EMBRYO (Diagrammatic). 
I. Longitudinal section of a developing ovum. ‘The folding off of the embryo has commenced, and the head fold 
bending down in front, has invaginated the amniotic area. The tail fold is partly formed, and the 
primitive alimentary canal, closed in front, communicates freely with the yolk-sac by a wide umbilical 
aperture. 
II. Transverse section of a developing ovum showing the commencement of the “folding off.” The thickened 
embryonic area is convex externally, and it already appears to sink below the surface of the ovum. 
III. Transverse section showing the ‘folding off’? more advanced. The changes seen in II. are more 
marked, and by apparent constriction at the junction of the embryonic area with the rest of the 
blastodermic vesicle the embryo is still further nipped off, and distinct lateral folds are formed. 
‘The division of the cavity of the blastodermic vesicle into that of the primitive alimentary canal and that of 
the yolk-sac is shown in all the figures. 
The amniotic area, directed upwards and inwards in II., forms with the placental area the amniotic fold, and 
in III. the amniotic folds of opposite sides are approaching one another over the back of the embryo to 
enclose the cavity of the amnion. The relative positions of the different areas of the blastoderm are 
correspondingly modified. 
AA, Amnion fold. EC. Ectoderm. PA. Placental area. SpM. Splanchnic mesoderm. 
C. Celom. EN. Entoderm. PAC. Primitive alimentary canal. YS. Yolk-sac. 
EA. Embryonic area. N. Notochord. SoM. Somatic mesoderm. Vea Walle 
time. In the latter case the protovertebral somites and the intermediate cell masses 
are solid; at a later period, however, a cavity which contains a few spherical cells 
appears temporarily in each somite. 
Folding Off of the Embryo from the Blastodermic Vesicle.—Although so 
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