ORGANIZATION AM) CELL-LINEAGE OF A.SCIDIAN EGG. 89 



fertilization. However, the evidence in favor of this is not conclusive as Wilson 

 admits. On the other hand. Boveri i L901 > has shown in the most convincing man- 

 ner that in Strongylocentrotus the polarity of the egg may be traced hack to the 

 ovocyte, and that this polarity determines the gastrular axis. It is. therefore, 

 possible that in all echinoderms the polaritj of the egg is predetermined in the 

 ovary, and not after the maturation and fertilization, and that in all cases the 

 maturation and ectodermal poles coincide. 1 



The most remarkable ami apparently well established of these exceptions to 

 the rule that the polar bodies are formed at the animal pole is that of the ascidians 

 studied by Castle (1894, 1896), where the polar bodies were said to he formed at 

 the vegetal or endodermal pole of the egg. However, this conclusion rests upon 

 erroneous orientation, as I have shown in the preceding pages ; in ascidians as in 

 other animals the polar bodies are formed at the ectodermal pole. There are, there- 

 fore, no well established exceptions to this general law. 2 



In many cases it is known that the polar differentiation of the egg may he 

 recognized while the egg is still in the ovary. Reference has just been made to 

 the condition in Strongylocentrotus in which the pole of attachment to the ovarian 

 wall becomes the maturation pole of the egg and the ectodermal pole of the larva. 

 Boveri says that in all known cases the pole toward which the germinal vesicle is 

 eccentric becomes the animal pole. In Unto, Lillie (1900) has demonstrated that it is 

 the free pole of the egg which becomes the maturation and ectodermal pole, while the 

 pole of attachment becomes the vegetal pole. In a number of gasteropoda (Lzmn&a, 

 Succinea. Polygyra, Limax, Phvsa. Planorbis. Ancylus) I have found that there 

 is a marked polar differentiation of the vfi'j: in the ovary, the germinal vesicle being 

 eccentric toward the tree pole of the ovocyte. 1 have elsewhere (1903) shown 

 reason for believing that in dextral snails the polar bodies are formed at the free 

 pole and in sinistral snails at the attached pole of the ovocyte. In his work on 

 Cerebratiihis, Wilson ( 1 9 >> > , found that the polar bodies were formed at the live 

 pole of the ovocyte, and again in his recent paper on Dentalimn (1904), he finds 

 the side of attachment in the ovary represents the lower or vegetal hemisphere. 

 We find then that the chief axis of the egg is very generally present in the ovocyte, 

 and that the free side usually uives rise to the maturation and ectodermal pole, 

 while the attached side becomes the vegetal pole; but in echinoderms and probably 

 also in sinistral gasteropods these conditions are reversed, the side of attachment 

 becoming the ectodermal pole. 



In the gasteropods named above, 1 found it possible to recognize this polarity 

 of the ovocyte at a very early stage ; in general it coincides with the "organic 

 axis" (Van Beneden), or the ''cell axis" (Heidenhain) i.e.. the axis passing through 

 the centrosome or sphere, and the center of the nucleus. This cell axis is a general 



1 However, Garbowski (1904) affirms that in Asterias (jlaeialis the polarity of the egg is net 

 determined even in the 8-eelI and 16-cell stages, and thai the blastomeres are equipotential up v> the 

 500-cell stag ' 



Wheeler 1897, p. 4 1 4t> has discussed in an admirable manner the apparent exceptions to 

 this law ni' polar differentiation and concludes that these exceptions are by no means well established. 



[2 JOUKN. A. X. S. IMIII.A.. VOL. XIII. 



