706 The American Naturalist. [August, 
schicht disappear by being incorporated in the true ectodermal 
layer, becoming at the same time columnar; this view is verified 
by Lieberkühn, 42, 400-401. As already stated, the rodent 
modification of the deckschicht is discussed below, page 711. In 
the rabbit the deckschicht disappears before the second layer of 
cells grows completely round the vesicle. 
Blastodermic Vesicles with Primitive Streak—The knot of Hen- 
sen becomes the front end of the primitive streak, which lengthens 
backward ; during the same period the vesicle as a whole en- 
larges; in ruminants the enlargement is enormous and very rapid.’ 
= The primitive streak always lies in the 
long axis of the shield. The formation 
of the primitive streak begins with the 
union of Hensen’s knot with the inner, 
layer, so that at the knot all three layers 
are actually united, —the condition origi- 
nally discovered by Hensen, 24, 268. 
The union of the knot with the inner 
layer spreads backward along the line 
which is to become the primitive streak ; 
soon the axial growth reaches the edge 
of the shield, and the streak and shield 
elongate together, the latter becoming 
Pid sd rn nic shield POinted at its hinder end. We thus have 
of a rabbit's ovum of five days, a pear-shaped shield, with the primitive 
een streak running forward from its pointed 
m', m"; after Kölliker, 2g end; the anterior end of the primitive 
Ren streak is somewhat enlarged, and the 
posterior end is considerably thickened; the three layers are 
united along the primitive streak. Figure 19 represents the 
embryonic shield of a rabbit embryo; the shield measured 1.34 
mm. in length, and 0.85 mm. in width; the primitive streak is a 
broad band, corresponding to the axial thickening, and extends 
about two-thirds of the length of the shield; the middle layer, 
m’, m", occupies a circular area around the hind end of the streak ; 
! Bonnet states that in the sheep the blastodermic vesicles must elongate during this 
period at the rate of one centimetre an hour. 
