PLATE IV. 
Figure 29. — Yiew of the nutritive pole of an egg a few 
hours older. 
Figure 30. — Yiew of the formative pole of a still older egg. 
Figure 31. — Optical vertical section of a somewhat older 
egg, figured with the polar globule above and the ectoderm to 
the right. The egg is now flattened from above downwards, 
and is disc-shaped in a surface view. The macromere has 
..given rise to a layer of larger granular cells, which are pushed 
in so as to form a large cup-shaped depression. The more 
transparent ''ectoderm, ec, now carries a few short cilia scat- 
tered irregularly, and the two layers are separated from each 
other by a segmentation cavity. This figure is in Plate III. 
Figure 32. — Surface view, and 
Figure 33. — Optical section of the embryo at the first swim- 
ming stage. The ectoderm has folded upon the endoderm, so 
as to form a primitive digestive cavity, with an external open- 
ing, g. The cilia of the velum have now made their appear- 
ance around the area occupied by the polar globule. This 
was not present in the egg from which the figure was drawn, 
but it was seen in other eggs, and is shown in a later stage of 
another embryo, Figure ’6. 
Figure 34 and Figure 35. — Two surface views of the em- 
bryo shown in Figure 32. 
Figure 36. — An older embryo, in the same position as Fig- 
ures 32 and 33. The external opening of the primitive diges- 
tive tract has closed up, and the two valves of the shell have 
appeared in the place which it had occupied. The endoderm 
has no connection with the exterior, and no central cavity 
could be seen. 
Figure 37. — A somewhat older embryo, figured with its 
dorsal surface above. There is a large, central, ciliated diges- 
tive cavity which opens externally by the mouth, m, which is 
almost directly opposite the primitive opening, the position of 
which is shown by the shell, s. 
