156 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



probably for the sake of clearness, semi-diagrammatic in character. While 

 his photographic 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 

 sti'uctures. 



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 nnsegraented 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 bauds," 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 well as his interpretations. 



c. Descriptiox of Loot's "Xeural Segmexts." 



I shall now give an account of the conditions, as I have found them, 

 in the head region of a shark embryo with 6 to 6i somites.^ Tliis 



^ In his final paper Locy speaks of the "neural folds or ridges" as "divided 

 throughout their length into a series of segments." 



- I count the somites beginning witli 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). 



