MUSEUM OF COMPARATIVE ZOOLOGY. 149 
PLATE IIL. 
Fig. 1. Abnormal? egg of Z. parma, in which the groove-like constriction which 
generally encompasses the ovum and forms the first cleavage plane, 
is limited to a furrow at one pole, p. 
Ovum in which this furrow, p, has deepened, forming a slit. 
Same egg in an older stage. 
Ovum in 2-cell stage, the original connection between the two cells in 
the undivided part of the original ovum turned from the observer. 
This egg is seen at right angles to the plane in which Figs. 1-3 are 
drawn. 
“5. An ovum in 4-cell stage, in which we have two large segment spheres 
and two small. This condition is thought to be uncommon. 
“6. Ovum in 4-cell stage, in which the beginning of the furrow destined to 
divide each of the four cells is found on one side, inner side, of all 
cells, and does not take the form of a groove-like constriction reaching 
wholly about the blastomere. 
7. Ovum in 4-cell stage in which each of the four blastomeres is divided 
later into two of unequal size. 
8-12, Formation of the 8-cell stage. They first represent the formation of a 
4-cell from a 2-cell stage, and then the subsequent breaking away of a 
part of one segmentation or cleavage plane, so that two of the blasto- 
meres are reduced to one. This is thought to be abnormal, patholo- 
gical, or at all events unusual. 
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PLATE IV. 
Fig. 1. Gastrula of Echinarachnius, in which the archenteron, ach, has made its 
way to the ventral side of the body. Lateral view. 
«2. The same, ventral view. 
“8. An older stage in which the limestone rods, sp, have begun to form. 
Lateral view. 
“4, View of the last from anterior pole. ° 
“6. An older larva, showing the differentiation of the anterior lobe and the 
posterior rods or arms. Ventral view. 
“6. A slightly younger larva seen from one side. 
“7. Anolder larva seen from one side. 
“8. A larva older than the last, seen from the side. 
PLATE V. 
Fig. 1. Gastrula of Z. parma. Lateral view. (The cilia on the body are too 
faintly photographed.) 
“2. Older gastrula showing the “water-tube” at vp. Lateral view. As the 
larva is under slight pressure, the spicule or calcareous rod, spi, is 
slightly thrown out of position. 
