ORGANIZATION AM) CELL-LINEAGE OF ASCIDIAN EGG. 75 



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, hut the anterior end of the embryo, who can say what it is? 

 In the early development of Amphioxus 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. The result of such a determination shows that there is great similarity 

 among the lower Chordata in that the anterior limit of the neural plate is always 

 some distance retnovedfrom the animal pole. In ascidians the chief axis of the egg is 

 plainly marked out not only b}- 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. In 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 embryonic axis is not a simple one, i. 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. The 

 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 egg 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. XX VII-XXIX). 



The axial relations are not so evident in Amphioxus and amphibians, since 

 the animal and vegetal poles are not so clearly marked as in the ascidians. Hat- 

 schek (lSSl ) supposed that the animal pole of the egg in Amphioxus was ventral 



