280 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
EXPLANATION OF PLATES. 
All Figures were drawn with the aid of the Abbé camera lucida. The magnifi- 
cation is stated for each 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 the 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 D) will be found 
a cell similarly situated on the other side of the median plane (in quadrant B or C); 
this cell should receive the same exponents as its mate, and either the letter or 0 
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 57.2; 
the respective mates of D7-5 and D'^5 in Figure 57 would be called 07-5 and 0%, 
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. 
ABBREVIATIONS. 
ast. 9 = female astral sphere. ms'chy. = mesenchyme. 
ast. d = male astral sphere. ma. = musculature. 
cd. = chorda. n. = nervous system. 
cl. pol. = polar globule. prnl. = pronucleus. 
cl. pol» = first polar globule. prnl. 9 = female pronucleus. 
ecdrm. = ectoderm. pr’nl.d = male pronucleus. 
en’drm. = endoderm. 
