ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 73 
the posterior portion of the nerve cord is formed from the muscle cells (his “ neuro- 
muscular" cells). Furthermore, I am unable to find satisfactory evidence that 
the ectoderm which covers the muscle cells and closes the blastopore notch behind 
contributes to the formation of the nerve cord. Therefore, zt is probable that 
the entire central nervous system comes from the neural plate, which 15 a portion 
of the anterior lip of the blastopore. 
After having overgrown the muscle cells and closed up the posterior notch 
of the blastopore the ectoderm forms a pair of V-shaped folds (figs. 52-54), the 
apex of the V lying just behind the blastopore and the limbs diverging anteriorly 
and laterally. By the forward extension of these folds the neural plate is rolled 
up into a tube which is covered with a layer of ectoderm, in the manner character- 
istic of vertebrates. These folds аге at first V-shaped, but after they have extended 
around the anterior end of the nerve plate they inclose an oval area which is pointed 
behind (fig. 55). The folds close from behind forward and ultimately convert the 
entire neural plate into a tube, which retains a lumen in its anterior portion (the 
sense vesicle) and an opening to the exterior (the neuropore), but which contains no 
lumen back of the anterior end of the notochord (figs. 166, 167). That portion of 
the nervous system dorsal to the notochord and which contains no lumen 15 derived 
from those neural plate cells which belong to the dorsal hemisphere and which 
wn origin were intimately associated with the chorda cells; the anterior half 
of the enlarged portion of the nerve tube lying in front of the notochord (sense 
vesicle) ts derived from those cells of the neural plate which belong to the ventral 
hemisphere. As nearly as I can determine the anterior end of the neural plate lies 
about 30° above the original equator of the egg and 60° below the animal pole. 
The cephalic pole of the larva lies ventral to the anterior end of the neural plate 
but dorsal to the animal pole; therefore, the antero-posterior axis coincides neither 
with the egg axis nor with the equatorial plane but lies mid-way between the two. 
The egg axis is therefore not dorso-ventral in the larva but is, strictly speaking, 
postero-dorsal and antero-ventral in direction. Inasmuch as the forward shifting 
of the animal pole by which this position of the axes is brought about occurs at a 
late period in the development, and also for the sake of simplicity of expression I 
have, in accord with all my predecessors, described the egg axis as dorso-ventral in 
direction in all the early stages. 
VI. COMPARISONS WITH AMPHIOXUS AND AMPHIBIA. 
The remarkable differentiations apparent in the egg and early cleavage stages 
of ascidians, the relatively small number of cells present during gastrulation and 
organogeny, and the comparative ease and certainty with which the axial relations 
of the egg and embryo can be determined at all stages,—these conditions render 
the ascidian egg the most favorable in the whole phylum of the chordata for an 
exact study of the early development. In no other chordate has the cell-lineage 
been followed in detail up to the formation of definitive organ bases, and no where 
else in the phylum has it been possible to determine with the same degree of cer- 
tainty as here the relations of the axes of the egg to those of the gastrula and larva. 
10 JOURN. A. N. S. PHILA., VOL. XIII. 
