ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 15 
1. Axial Relations of Egg and Embryo. 
In considering the axial relations of egg and embryo one is confronted at once 
with the difficulty of determining what is meant by the anterior pole, unless it be 
defined in terms of structure rather than function. The animal pole is a structur- 
ally definite point, but the anterior end of the embryo,—who can say what it is? 
In the early development of Amphzoxus and ascidians the point which at one time 
is most anterior does not continue such for any considerable period, and it is prac- 
tically impossible to determine the exact point of this rounded anterior portion of 
the embryo which will become the most anterior part of the body. Not only is the 
animal pole a structurally definite point but the anterior limit of the neural plate 
is also, and the relative positions of these two can be determined with considerable 
accuracy. Тһе result of such a determination shows that there is great similarity 
among the lower Chordata in that the anterior limit of the neural plate ts always 
some distance removed from the animal pole. In ascidians the chief axis of the egg is 
plainly marked out not only by the well differentiated cleavage cells but also by the 
polar bodies which in some cases remain attached to the egg at the point of their 
formation until the blastopore has closed. Іп the early gastrula the animal pole is 
slightly posterior to the middle of the ventral hemisphere, the vegetal pole marks 
the middle of the dorsal hemisphere, and the deepest point of the gastrocoel (text 
fig; XXVIII). In the closure of the blastopore the anterior lip overgrows the 
‘archenteron, and the blastopore, being closed from in front backwards, is finally 
limited to a longitudinal groove in the posterior half of the dorsal face of the 
gastrula. 
The relation of the egg axis to the embryonie axis is not a simple one, 2. e., 
they neither coincide nor is one at right angles to the other. During the over- 
growth of the anterior lip the animal pole is shifted nearer to the anterior end of 
the gastrula. This may be, and probably is in part, due to a shifting of the point 
of greatest curvature at the anterior end to a point nearer the animal pole. Zhe 
anterior edge of the neural plate never reaches farther forward than about one- 
third of the way from the equator to the animal pole, and consequently the animal 
pole lies on the ventral side of the larva but near the head end. Correspondingly 
the opposite pole of the extended egg axis lies near the posterior end of the dorsal 
. side and consequently not far from the place where the last trace of the blastopore 
can be seen. 
Previous students of ascidian embryology, and particularly Castle and Samassa, 
have considered that the ege axis was dorso-ventral and hence perpendicular to the 
embryonic axis. I at first held the same opinion, but observations on the change 
of shape of the gastrula and particularly upon the anterior limit of the neural 
plate during the closure of the blastopore have convinced me of the truth of 
the position here taken (cf text figs. XXVII-X XIX). 
The axial relations are not so evident in Amphzoxus and amphibians, since 
the animal and vegetal poles are not so clearly marked as in the ascidians. Hat- 
schek (1881) supposed that the animal pole of the egg in Amphioxus was ventral 
