20 
T be Embryology of Patella. 
outer walls of the embryo and the mesenteron. They probably 
form the blood corpuscles. 
I was not able to determine what beeame of the primitive 
mesoderm cells which at the 12Ü th hour, the oldest embryo care- 
fully studied, were still distinguishable from the other mesoderm 
cells by their greater size. 
As soon as the foot becomes hollowed within, it is also filled 
with scattering mesoderm cells. 
The sliell gland, which at the end of the preceding stage 
had only formed a thick plate of cells slightly hollowed in the 
centre, dnring the present period assumes such enormous pro- 
portions as to change the shape of the whole posterior portion of 
the embryo and to exert a modifying influence upon the processes 
which take place within the interior of the embryo. 
The cells become very deep as the invagination of the gland 
progresses. The cavity of the gland is at first shallow and wide 
mouthed, but gradually decreases in size still retaining its cir¬ 
cular opening; Pl. V, Fig. 59. When the latter has reached its 
smallest limits it loses its circular outline, increases in size and 
becomes laterally extended, at the same time the cavity itself, 
which has meantime increased in depth, becomes roughly Y shaped, 
Pl. IV, Fig. 46 and Pl. V, Fig. 64. The shell gland, the cavity of 
which is now surrounded with very thick walls. occupies almost 
the entire dorsal surface of the embryo posterior to the velum, 
and soon encroaches upon its lateral walls; the floor of the 
cavity decreases rapidly in thickness while its edges become more 
prominent. 
This decrease in the thickness of the bottom of the cavity 
is due to its lateral and posterior extension, which in time causes 
the anterior arm of the Y shaped cavity to disappear. Pl. V, 
Fig. 57 and 65. The bottom of the cavity is thus entirely exposed 
to the outer world, the only trace of it which remains is the 
posterior arm of the Y, which still forms an infolding of the ecto- 
derm at the posterior end of the embryo. The lips of the in¬ 
vagination, which has thus become almost entirely obliterated, 
still retain their great size and form a gradually widening ring 
which grows toward and finally encircles the ventral surface of 
the embryo. This thickened ring forms the edge of the mantle. 
The shell itself, which at first formed only a thin membrane 
over the opening of the gland, increases in thickness and becomes hat- 
shaped and slightly corrugated at its apex; Pl. V, Fig. 57. The 
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