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embryo. However, most of the figures illustrating this article are 

 enlarged from photographs, and I have made no diagramatic altera- 

 tions, but have represented what shows fairly on the photographs. 

 The metameres are most clearly seen from below, in fact, they are 

 rather difficult to ob- 

 serve from above, but , 

 the reason for this 

 fact is not difficult to 

 find. It will be noted, 

 in this figure, that 

 the lateral margin on 

 each side is separated 

 from the median part 

 of the medullary plate 

 by a furrow running 

 lengthwise of the body. 



Fig. 2. Young embryo of Squalus acanthias, showing the two marginal bands in 

 which metamerism is clearly expressed. The line AA 1 is drawn just in front of the future 

 origin of the vagus nerve. There are 11 pairs of segments in front of it. Figs. 1 and 

 2 are prior to the formation of medullary folds, the furrow in the middle line of the 

 embryo is not the medullary groove. 



The furrow is very distinct in the head region, but, it extends also with 

 less distinctness through the trunk region. In this way, marginal 

 bands are marked off that run the entire length of the embryo, and 

 the metameric segmentation is most distinct in these bands which 

 represent, I think, the rudiments of the dorsal nerve cord. Sections 

 show that these marginal bands, at this stage, are composed of thick 

 groups of cells, and the neural plate between them is thinner. 

 These bands of cells and their immediate derivatives form the 

 material out of which the medullary folds are straightway produced. 

 Now the conditions of the medullary folds in this animal are very 

 unusual; when first formed, they are lateral, wing-like, expansions 

 extending along each side of the embryo and overhanging the yolk. 

 No sooner are they formed than they become ventrally curved, and, 

 in this way, the most clearly segmented parts of the embryo are 

 brought ventralwards, and this accounts for the metameres being 

 much more distinct when viewed from the ventral surface. 



Figs. 3 and 4 represent two views of the same embryo. It is 



