72 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



in the trunk of the larva, and they may therefore be known as trunk mesenchyme 

 cells ; in figures 161-165 they. consist of eight or ten cells on each side. Whether 

 there may be a few scattered mesenchyme cells between the caudal and the trunk 

 groups and ventral to the muscle rows must still be left an open question, but there 

 can be no doubt that most of the mesenchyme cells are located in these two groups. 

 The separation of the caudal from the trunk mesenchyme must have been accom- 

 plished in part by the same means which brought the muscle rows from a vertical to 

 a horizontal position, viz., by the outgrowth of the tail. In addition there seems to 

 have been an actual forward movement of the trunk mesenchjme, as is indicated 

 by a comparison of such figures as 156 and 161. This is probably part of the general 

 forward shifting of the animal pole. In later stages when the tail is bent toward 

 the ventral side, the trunk mesenchyme is found ventral to the anterior ends of the 

 muscle rows (figs. 59, 166, 167). In these later stages the mesenchyme cells are 

 frequently found dividing; they are smaller and more numerous than the muscle 

 cells and are more than one cell-layer thick. 



In the formation of the larva the ventral cord of endoderm increases greatly in 

 length, being composed in very young tadpoles (fig. 161) of six or seven pairs of cells. 

 These cells form a double row between the muscle cells of each side and ventral to 

 the notochord. In front of the caudal endoderm and notochord lies the gastral 

 endoderm consisting of yolk cells which form a single but rather irregular layer 

 around a small central cavity, the enteron (figs. 161, 162, 164-166). 



In young larva 1 the chorda cells are wedge-shaped and form two or more rows 

 of cells which interdigitate, as has been described by previous writers. In the 

 latest stage which I have studied (fig. 167) these cells interdigitate to such an extent 

 that the}- form a single row of disk-shaped or slightly wedge-shaped cells. I have 

 not followed in detail the method by which the two arcs of chorda cells shown in 

 figure 153 are transformed into the double row shown in figure 162, but I see no 

 reason to question the account given of this by Van Beneden and Julin and also 

 by Castle. 



The neural plate grows backward with the notochord nearly to the posterior 

 end of the embryo. I can find no evidence in favor of the view that any portion 

 of the nervous system is derived from cells which bound the blastopore groove pos- 

 terior to the neural plate (figs. 152, 153), nor is there any evidence for the existence 

 of a nerve ring surrounding the blastopore. Since the neural plate, six or eight 

 cells wide at its hinder end, is carried back with the chorda nearly to the hinder 

 end of the embryo where the last trace of the blastopore is found (fig. 53), and since 

 no portion of the nerve cord is found posterior to the blastopore and notochord 

 (figs. 163, et seq.), it seems most probable that the hinder portion of the nerve 

 cord, as well as all the rest of it, is derived from the neural plate and not from 

 the lateral lips of the blastopore groove. That the muscle cells do not give 

 rise to the posterior part of the nerve cord, as claimed by Castle, is made probable 

 by the fact that this portion of the nerve cord is not yellow, as are the muscle 

 cells: I cannot therefore accept without further evidence Castle's statement that 



