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THE POPULAR SCIENCE MONTHLY 



Thus within a few minutes after the fertilization of the egg, and 

 before or immediately after the first cleavage, the anterior and posterior, 

 dorsal and ventral, right and left poles are clearly distinguishable, and 



1P.S. 



& 



Pig. 29. Sections of the Egg of Styela, showing maturation, fertilization and 

 early cleavage; 1 P. 8., first polar spindle, p. b., polar bodies, tfN, sperm nucleus, %N, 

 egg nucleus, p. I., peripheral layer of yellow protoplasm, Cr, crescent of yellow proto- 

 plasm, A s , A 3 , anterior cells, B 3 , B s , 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 char- 

 acteristic positions. 



