62 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
cells in the tail of the tadpole (figs. 58, 59), and so far as I am able to determine do 
not contribute anything to the nervous system. In general the mesenchyme cells 
may also be traced by their faint yellow color until they give rise to the mesen- 
chyme of the tadpole, and, with the exception of the small cell which Castle calls 
the * posterior chorda fundament," I agree with him as to the fate of these cells. 
Castle figures and describes the cell, A**, as the most anterior of the mesenchyme 
cells. I do not find that it contains yellow protoplasm in Cyzz/a, but its histolo- 
gical structure is different from that of the other endoderm cells; I shall therefore 
follow Castle in classing it as a mesenchyme cell. The median cells of the crescent 
(В?) resemble in their deep yellow color, the muscle cells rather more than the 
mesenchyme, and Castle reckons these cells with the * neuro-muscular ring," but 
the later history of these cells shows that they lie just beneath the notochord and 
at the hinder end of the ventral endodermal cord in the tail of the tadpole (figs. 
161-165); therefore, they do no give rise to the lateral muscles, and they are pro- 
bably to be counted as mesenchyme cells. 
At this stage, therefore, the endoderm consists of four pairs of cells meeting 
along the mid-line (figs. 154, 200), and of one pair of laterally placed cells which lie 
just in front of the second cleavage plane; the chorda consists of an arc of eight cells 
bounding the endoderm in front; in front of the chorda cells and below the equator 
is an arc of eight neural plate cells. Posteriorly the endoderm is bounded by an 
are of twelve mesenchyme cells, while just outside these is an are of six muscle 
cells (eight, in fig. 154). Тһе neural arc in front 18 separated from the muscle are 
behind by the most dorsally situated of all the ectoderm cells (b). But for this 
lateral interruption it would be possible to speak of a “ neuro-museular ring" as 
Castle does. Тһе chorda and mesenchyme ares form a continuous chorda-mesen- 
chyme ring, as Castle has shown. 
Castle asserts (1896, p. 246) that the mesenchyme *is made up of cells 
derived from both hemispheres and all four quadrants," and again that two cells 
“viz. d^? and с? [my БУ are the sole contribution of the dorsal hemisphere to 
the mesoderm of the larva” (p. 242). This is certainly not the case; the mesen- 
chyme and muscle cells are derived entirely and exclusively from the dorsal hemi- 
sphere and largely from the posterior quadrants. The most posterior cells of the 
crescent on each side (В®' and its mate) are counted by Castle as part of the ecto- 
derm; their histological structure, color in the living egg of CyzzZza and later 
` history show that they are the most posterior of the muscle cells. Of two other 
cells of this stage, Castle says (p. 242), “ it is noticeable that 4% and its mate сб 
have been shoved forward out of their own quadrants to a position beside the endo- 
derm cells derived from the anterior quadrants.” These cells are really А? and 
its mate, as is shown by their origin and later history (figs. 120, et seq.), and do not 
belong to the posterior but to the anterior quadrants. EE 
The 76-cell stage is of very short duration, for immediately after thos... 
sions in the dorsal hemisphere which advance the embryo from the 64-cell to the 5%, 
76-cell stage, all the cells of the ventral hemisphere divide simultaneously. The 
