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 
а horizontal position, vzz., by the outgrowth of the tail. In addition there seems to 
have been an actual forward movement of the trunk mesenchyme, as is indicated 
by a comparison of such figures as 156 and 161. Thisis 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 larvz 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 they form a single row of disk-shaped or slightly wedge-shaped cells. І have 
not followed in detail the method by which the two ares 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. a 
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. 169, е/ seg.), 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 
CS S ds д ME s = am 
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