PHALLUSIA MAMILLATA 401 



sometimes very difficult to determine whether such | larvae are 

 entire in the region of the "head" (cf. fig. 10); as just said the 

 gastric wall and body wall have completely closed on the injured 

 side and it is only by studying some typically paired structure 

 such as the mesenchyme areas in which the atrial cavities later 

 form, that the incomplete character of the larva can be seen. In 

 the region of the tail, however, sections show at once whether 

 the larva is entire or not, for here three rows of large muscle cells 

 lie on each side of the notochord in typical larvae (fig. 7). On 

 the other hand in | larvae three rows of muscle cells are found 

 on one side only of the notochord (figs. 6b, 8, 10, 12), and in not a 

 single instance have I found any evidence that muscle cells occur 

 on the other side. In a few instances four muscle cells may be 

 seen in section of the tail, as in fig. 10, but such cases are almost 

 invariably due to obliquity of the section. That these | larvae 

 are really not entire is most convincingly shown in cases, such 

 as are figured in 6b and 8, where the nerve chord of the mid- 

 dorsal line and the caudal endoderm of the mid ventral line come 

 into contact on the injured side, while the muscle cells lie entirely 

 on one side of the notochord. To quote the words of a former 

 paper (1906, p. 743)," These | larvae are exactly such as would 

 result if a fully formed larva were cut in two along the median 

 plane and the cut edges of each half then came together, the 

 dorsal and ventral mid lines joining." The parts peculiar to the 

 missing half are not restored, although the larvae may appear, 

 superficially, entire. 



So far as my experience goes neither anterior nor posterior | 

 blastomeres ever give rise to forms even superficially resembling 

 typical larvae. In these forms there is no distinction of body 

 and tail, no typical notochord, gastric cavity, neural tube, nor 

 muscle rows. On the other hand the chorda cells, neural plate 

 and eye spots are found only in the anterior half, the muscle cells 

 only in the posterior half and these cells do not have their typical 

 arrangement in rows. Each half produces only that histological 

 type of cell which it would have produced in a typical larva, and 

 there is no evidence that missing parts or tissues are ever restored 



THE JOURNAL OF EXPEHIMENTAl, ZOOLOGY, VOL. 10, NO. 4. 



