156 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
probably for the sake of clearness, semi-diagrammatic in character. While 
his photographie reproductions show absolutely no segmentation in the 
early stages, his drawings on the contrary show in these same stages 
“segments” as clearly marked as those of later stages. Photography 
is obviously unsatisfactory as a means of reproducing these delicate 
structures. 
Before taking up the consideration of the evidence which I have obtained 
from my studies, it is well to give a brief review of Locy’s results. In 
his final paper (95) he qualified his statement that the segmentation is 
solely epiblastic, since he discovered in sections that it may be found 
in both mesoderm and ectoderm. He therefore concludes that the 
segments seen in surface study are the remnants of a primitive metam- 
erism of the Vertebrate body. ‘The more important points in Locy’s 
description may be briefly summarized as follows. The evidence of seg- 
mentation appears first in the non-axial part of the embryo, i. e. along 
the thickened blastodermic rim. The segments later extend along the 
lateral margin of the neural plate from the anterior unsegmented tip of 
the embryo backward into the non-axial part. The segments are most 
clearly seen in “marginal bands along the neural plate,” though “in 
the trunk region the lines of division may be traced inward toward the 
median furrow. This is probably due to the appearance of the meso- 
dermic somites in that region.” The “marginal bands,” he thinks, 
“represent the dorsal nerve cord.”* “These segments, once estab- 
lished in this very early stage, may be traced onward in an unbroken 
continuity until they become the neuromeres of other observers, and 
sustain definite relations to the spinal and cranial nerves.” In the con- 
clusion of his preliminary paper Locy writes, “No one is likely to ques- 
tion but what the segmented condition I have described represents a 
survival” (i.e. of an ancestral segmentation 1). My own observations 
on embryos of S. acanthias lead me to question in large part the accu- 
racy of Locy’s observations, as weli as his interpretations. 
c. Description or Locy’s “ Nnurat Suements.” 
T shall now give an account of the conditions, as I have found them, 
in the head region of a shark embryo with 6 to 61 somites.? This 
+ In his final paper Locy speaks of the “neural folds or ridges ” as “ divided 
throughout their length into a series of segments.” 
2 T count the somites beginning with van Wijhe’s 7th somite (somite 7 of my 
figures), the myotome of which becomes the first segment of the lateral trunk 
musculature (van Wijhe). 
