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, it is probable that 

 the entire central nervous system conies from the neural plate, which is 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 Y-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 are at first V-shaped, but after they have extended 

 around the anterior end of the nerve plate the}' 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. 16G, 167). That portion of 

 the nervous system dorsal to the notochord and which contains no lumen is derived 

 from those neural plate cells which belong to the dorsal hemisphere and which 

 in origin ivere intimately associated with the chorda cells ; the anterior half 

 of the enlarged portion of the tierve tube lying in front of the notochord (sense 

 vesicle) is derived from those cells of the neural plate which belong to the ventral 

 hemisphere. As tiearly as I can determine the anterior end of the neural plate lies 

 about jo above the original equator oj the egg and 6o below the anitnal 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 he 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 hases, and no where 



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 JOUEN. A. X. S. PHILA., VOL. XIII. 



