280 bulletin: museum of comparative zoology. 



EXPLANATION OF PLATES. 



All Figures were drawn with the aid of the Abbe camera lucida. The magnifi- 

 cation is stated for eacli plate separately. The egg membranes have not been rep- 

 resented in any of the Figures. Arrows in the Figures connect sister cells, i. e. cells 

 which have arisen by division of a common mother cell. For an explanation of the 

 system of nomenclature employed, see page 226. 



In many of tlie Figures, only the cells of one half of the embryo (usually the 

 right, that is, quadrants A and D) have been lettered. The reader will be able 

 readily to supply the deficiencies for the cells of the other half, since they are 

 almost perfectly symmetrical in position with those that are lettered. Correspond- 

 ing to each cell on one side of the median plane (in quadrant A or IJ) will be found 

 a cell similarly situated on the other side of tiie median plane (in quadrant B or C) ; 

 this cell should receive the same exponents as its mate, and either the letter B or C 

 according as that mate is lettered A or D. Thus, the mate of A'-'' in Figure 58 

 (Plate X.) is the cell immediately to the right of it, which would be called B^-^; 

 the respective mates of !>••* and D'''^ in Figure 57 would be called C'-* and C-^. 



In Figure 62 (Plate X.) and all following Figures, the endoderm cells are indi- 

 cated by granular nuclei in a cell body that is left without tint or stippling ; the 

 chorda cells without tint or stippling, and with the outlines only of nuclei ; the 

 mesenchyme cells by a flat tint ; the ring of neuro-muscular cells by stippling of 

 the body of the cell. The ectoderm cells, since they are easily distinguishable 

 from the endoderm cells, have been left, like the latter, without stippling or tint ; 

 their nuclei are sometimes drawn true to nature, i. e. with granulations, sometimes 

 in outline only. 



