262 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



to form the medullary plate. Those cells of this neuro-muscular ring 

 which lie on each side of and posterior to the blastopore are for the 

 most part invaginated, and form the entire longitudinal musculature of 

 the tail. Some of them, however, form the most posterior portion of 

 the nerve cord. 



3. Lying just within the margin of the blastopore, and encircled by 

 the neuro-muscular ring, is another ring of cells, interrupted at the pos- 

 terior end of the embryo only. Its anterior portion gives rise to the 

 greater part of the chorda ; its remaining (lateral) portions produce the 

 mesenchyme or trunk mesoderm, besides contributing to the chorda a 

 single cell at each lateral margin of the blastopore. The descendants 

 of these two chorda cells meet in the median plane at the closure of the 

 blastopore. They form the most posterior portion of the chorda. 



We may regard the chorda-mesenchyme ring as being completed mor- 

 phologically by the two small sub-chordal mesoderm cells C' b , Z> 75 , which 

 have been wedged in between the most posterior cells of the neuro- 

 muscular ring. Like the other cells of the chorda-mesenchyme ring, 

 they lie in contact with the endoderm cells on one side, and with cells of 

 the neuro-muscular ring on the other. Ultimately they probably form 

 mesenchyme in the tail region. Possibly by a ccenogenetic reduction 

 in size to their present minute dimensions, a gap has been left on each 

 side of the embryo between them and the lateral portions of the chorda- 

 mesenchyme ring. This change may have attended a ccenogenetic 

 lengthening of the posterior end of the organism to subserve locomotion. 

 There is evidence from other sources that the trunk of Ascidians formerly 

 extended farther back into what is now the tail region of the larva. At 

 that time the mesenchyme also probably extended farther back, and the 

 chorda-mesenchyme fundament was in ontogeny, as we suppose it to 

 have been in phylogeny, an uninterrupted ring. 



4. The blastopore, at first widely open, closes more rapidly from the 

 anterior margin and from the sides than from behind. Consequently it 

 comes to lie in the posterior portion of the dorsal surface of the embryo, 

 and is triangular in form. The right and left sides of the triangular blas- 

 topore, however, fuse from behind forward, beginning in the region of the 

 pair of small, flattened mesoderm cells, O'- 5 , D 75 . Along the line of union 

 of the lateral lips of the blastopore lies superficially on each side of the 

 median plane a row of nerve cells. These are subsequently covered in 

 by ectoderm from the sides and from behind, and form the posterior por- 

 tion of the nerve cord. Underneath them, and at first not distinguishable 

 from them in histological characters, are other cells, likewise derived from 



