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THE POPULAR SCIENCE MONTHLY 



approximate positions and proportions which they will have in the 

 embryo and larva (Figs. 28-31). 



Indeed, there are types of localization of these cytoplasmic materials 

 in the egg which are characteristic of certain phyla; thus there are the 

 ctenophore, the flat-worm, the echinoderm, the annelid, mollusk and 

 the chordate types of cytoplasmic localization (Fig. 45). The polarity, 

 symmetry and pattern of a jelly fish, star-fish, worm, mollusk, insect or 

 vertebrate are forshadowed by the characteristic polarity, symmetry and 



Fig. 43. Invebse Symmetry of the 3d, 4th. 5th and 6th cleavages. The 

 cells la-Id, 2u-2d and 3a-3rf give rise to all the ectoderm ; 4tf or M gives rise to meso- 

 derm ; Aj B, Cj D to endoderm. 



pattern of the cytoplasm of the egg either before or immediately after 

 fertilization. In all of these phyla eggs may develop without fertiliza- 

 tion, either by natural or by artificial parthenogenesis, and in such cases 

 the characteristic polarity, symmetry and pattern of the adult are 

 found in the cytoplasm of the egg just as if the latter had been fertil- 

 ized. The conclusion seems to be justified that these earliest and most 

 fundamental differentiations which distinguished the eggs of various 

 phyla are not dependent upon the sperm. 



