Page:Popular Science Monthly Volume 85.djvu/122

118 before or immediately after the first cleavage, the anterior and posterior, dorsal and ventral, right and left poles are clearly distinguishable, and

 Styela, showing maturation, fertilization and early cleavage; 1 P. S., first polar spindle, p. b., polar bodies, &#x2642;N, sperm nucleus, &#x2640;N, egg nucleus, p. l., peripheral layer of yellow protoplasm, Cr, crescent of yellow protoplasm, A3, A3, anterior cells, B3, B3, posterior cells of the 4-cell stage. In 1 the sperm nucleus and centrosome are at the lower pole near the point of entrance; in 2 and 3 they have moved up to the equator on the posterior side of the egg; in 4 the egg and sperm nuclei have come together and the sperm centrosome has divided and formed the cleavage spindle; in 5 the egg is dividing into right and left halves in 6 it is dividing into anterior and posterior halves.

the substances which will give rise to ectoderm, endoderm, mesoderm, muscles, notochord and nervous system are plainly visible in their characteristic positions.