127 
ably indicates considerable activity in the yolk of this region. In the 
peripheral yolk region adjoining the recently formed dorsal lip, I have 
not found a marked difference in shape between the marginal and more 
centrally situated cells. | 
In watching the gradual approximation between blastopore lip and 
yolk cells, both in the case of dorsal and ventral lips, I have some- 
times been able to note a very obvious change in the shape of the 
yolk cell, as the lip drew near it, while in other cases the yolk cell 
exhibited as far as I could see, absolutely no change of shape. I am 
inclined to believe, then, that there is no uniformity in the behavior 
of the yolk cells as the blastopore lip approaches them: they may or 
may not undergo a change of shape during the period in which they 
lie near but not quite at the edge of the yolk plug. Of course when 
they reach the extreme edge and begin to involute and to be covered 
by the advancing lip, they doubtless change shape in some measure, 
but as regards this change the only evidence obtainable seems to be 
sectional evidence. 
The fact that in the normally placed egg the dorsal and ventral 
lips of the blastopore behave alike, is it seems to me in flat contra- 
diction with the concrescence theory of the formation of the frog 
embryo. In the Chorophilus embryo the neural plate when first 
formed, minus the anterior connective, has an angular length of fully 
120°. The entire white surface extending between the dorsal and 
ventral blastopore lips, in the positions where they are first differen- 
tiated, measures 120° or close to it. Now if the tissue composing the 
neural plate were really produced by a concrescence from the dorsal 
lip backwards, of the lateral blastopore lips, the dorsal lip should 
travel 120° over the yolk, while the ventral lip should remain abso- 
lutely stationary, or granting the probability of slight errors in my 
measurements, nearly stationary as maintained by MorGAn (Quart. 
Journ. Micr. Sc., Vol. 35). But the fact seems to be that the ventral 
lip is far from stationary, in that directly upon its formation 
it begins to overgrow the yolk at about the same rate 
as the dorsal lip. My computation (1. c. p. 228) is that the dorsal 
lip moves approximately 25° over the yolk before the ventral lip is 
distinctly outlined; and at the time when the ventral lip is differenti- 
ated, the angular distance between the two lips is approximately 95°. 
Since the rate of overgrowth is about the same for the two lips, it 
follows that the total overgrowth on the part of the ventral lip must 
be in the neighborhood of 471/,°. According to this calculation the 
conclusion is that the anterior part of the neural plate (anterior con- 
