328 WHEELER. [Vou. IIT. 
from the anterior pole. It pushes its way through the yolk in 
such a manner as not to bring the amnion or serosa in close 
apposition, but to inclose a greater or less amount of yolk 
between the two membranes. This dorsal growth of the caudal 
end shows considerable variation, however. In some eggs the 
end of the tail comes to lie at such a distance from the dorsal 
surface that it is almost in the centre of the yolk. In other 
instances the amnion and serosa become closely apposed and 
extrude the yolk from between them soon after the two enve- 
lopes have closed over the mouth, and the embryo has attained 
its maximum length. 
In Fig. 71 I have represented the much curved embryo in this 
stage straightened out. The gastrular invagination has not yet 
closed, but it is much narrowed and more lengthened on account 
of the greater growth in length of the embryo. The oral end 
is still wider than the remaining portion. At the anal end (Fig. 
71 x) the groove seems to bifurcate. At the point x the procto- 
dzeum is subsequently invaginated. Usually in embryos as old 
as that represented the anterior half of the gastrula has closed 
completely. 
In the embryo figured the cephalic, maxillary, thoracic, and 
abdominal portions are already marked out. The first begin- 
nings of the three pairs of legs are apparent in the undulating 
edge of the thoracic portion of the ventral plate (p! 7? f°). » 
A median cross-section of the egg in the stage just described 
cuts the embryo at right angles to its long diameter in two 
places (Fig. 65). At the posterior end the thick amnion (az) 
is separated from the serosa by a layer of yolk (y), which will 
shortly be pushed out from either side to join the great central 
yolk mass. In this half of the section which is near the caudal 
end the gastrula is still open, though its cells have ceased to be 
columnar, and are dividing rapidly to form the thick lump of 
cells so very conspicuous at this point in slightly later stages. 
In the anterior portion of the egg the gastrula has closed, and 
the ectoderm and mesoderm are clearly separated. The tubular 
walls of the gastrula have broken down to form an irregular 
mass of polygonal cells which lie in the median line closely 
applied to the outwardly convex ectoderm. Though the clos- 
ing of the gastrula progresses from before backwards, the 
closure of the oral portion is retarded and is marked by a 
