116 The Development of Moina. [ February, 
The eggs of Moina are oval and measure about one-tenth of 
a millimetre in length. Segmentation occurs first on the minor 
and then on the major axis, thus producing four cleavage spheres. 
A polar view of these is shown in Fig. 1, the food granules 
(deutoplasm) being seen between the blastomeres. Another divi- 
sion produces eight segments, but after this, cleavage takes place. 
at unequal rates in different parts of the egg. As we understand 
the matter, these cleavage planes do not all pass completely 
through the yolk. In the resulting mulberry stage the “ genital 
cell” (which afterwards produces the genital system) is differen- 
tiated even before the first signs of the gastrula stage. (Fig. 2 g, 
genital cells or the cells which by invagination produce the endo- ° 
derm. In these cells the so-called Amphiaster is shown.) Soon 
there begins an invagination of the endodermal cells, and other cells 
arise beneath the ectodermal cells near the genital cells. These form 
the mesoderm. Others, opposite the genital cells, are larger, and 
these form the cephalic germinal plate, “ scheitel platte.” (Fig. 3, 
an optical section; ec, ectoderm ; ms, mesoderm; g, genital cells; 
£g m, gastrula mouth; ez, the cells which form the endoderm ; 
c, cephalic germinal plate; D, deutoplasm.) Then the gastrula 
mouth closes up, the genital and endodermal cells become inter- 
nal, the body is constricted and a fold is formed on either side, 
the first indication of appendages, which develops into the second 
antenna. A depression occurs in the ectodermal cells, where 
finally the mouth appears. (Fig. 4, lettering as before; a”, second 
antenne ; m, depression for mouth.) In the next phase two more 
appendages have budded, the antennulz and mandibles, while the 
antennal bud becomes two-lobed. This constitutes the nauplius 
stage. (Fig. 5, a’, antennulz; md, mandible.) Next, two of the 
thoracic feet are indicated, then~two more, the brain begins to 
appear, the mouth depression is deeper, the anus is indicated and 
the genital cells divide and are placed on each side of the median 
line. Then the maxillæ and the fifth pair of feet appear; the eye 
begins to be developed from the ectodermal cells, the shell gland 
is present and the abdominal bristles are outlined. (Fig. 6, 0’ 0”, 
brain ; ma! m2, first and second maxilla; /'-7°, thoracic feet; 7, 
branchial sacs; a4, abdominal bristles; æ, anus ; s, shell gland; 
v, vitelline membrane; other letters as before.) The succeeding 
changes are those of increase in size of the appendages, in which 
joints appear, the formation of the shell, appearance of heart and 
the covering of the eye, and at last the animal acquires the perfect 
form (Fig. 7, from Baird), -> : | Pur 3 
