02 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



cells in the tail of the tadpole (figs. 58, 59), and so far as 1 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 70 , as the most anterior of the mesenchyme 

 cells. 1 do not find that it contains yellow protoplasm in Cynthia, 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 

 (B 7S ) 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. 134, 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 

 arc of twelve mesenchyme cells, while just outside these is an arc of six muscle 

 cells (eight, in lig. 134). The neural arc in front is separated from the muscle arc 

 behind by the most dorsally situated of all the ectoderm cells (b 817 ). But for this 

 lateral interruption it would be possible to speak of a " neuro-muscular ring" as 

 Castle does. The chorda and mesenchyme arcs form a continuous chorda-mesen- 

 chynie 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 

 " vis. d' '- and c 712 [my B 85 ] 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 (B S1,; and its mate) are counted b\ Caslle as part of the ecto- 

 derm; their histological structure, color in the living egg of Cynthia and later 

 history show that they are the most posterior of the muscle cells. Of two other 

 cells of tbis stage, Castle says (p. 242). " it is noticeable that d 65 and its mate c 65 

 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 A 75 and 

 its mate, as is shown by their origin and later history (figs. 120, et set/.), and do not 

 belong to the posterior but to the anterior quadrants. 



The 76-cell stage is of very short duration, for immediately after those divi- 

 sions in the dorsal hemisphere which advance the embryo from the 64-cell to the 

 76-cell stage, all the cells of the ventral hemisphere divide simultaneously. The 



