L06 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



they give rise. He calls particular attention to the fundamental resemblances 

 between the eggs of Dcntalium and of Mysostoma'va the matter of the polar 

 lobe and the " pillar of protoplasm." Furthermore this lobe is comparable to 

 the polar rings of leeches and oligochastes. Such a lobe, although present in some 

 annelids and mollusks, is not present in all of them, and this would at first thought 

 seem to mark some important difference in localization. But the presence or 

 absence of such a lobe probably indicates no fundamental dissimilarity in the locali- 

 zation, but rather variations in the surface tension and fluidity of different eggs. 

 Although there are many interesting differences between various annelids and 

 mollusks in the size of the polar lobe, of the blastomeres and of larval organs, these 

 differences mark variations in the proportions of parts rather than in the type of 

 localization. In all known cases among annelids and mollusks corresponding organs 

 arise from corresponding regions of the egg. 



It may be concluded also from the work of Wilson (1903) and Yatsu (1904) on 

 Cerebratulns that the character of the localization in the nemertine egg is essentially 

 like that of the annelid and mollusk, though many of the details of localization 

 are less accurately known in this case than in the others named. 



2. Ctowphore Type. 



If Fischel (1903) is right regarding the localization which he ascribes to 

 the unsegmented ctenophore egg there is one fundamental difference between the 

 ctenophore and other animals whose types of localization are known. On the 

 authority of Metschnikoff he derives the mesoderm (somewhat doubtfully it must 

 be said) from the micromeres at the upper pole of the egg, and consequently in his 

 fig. 21 (p. 708) he localizes the mesodermal material at the upper pole of the unseg- 

 mented egg. A zone beloAv this, reaching to the equator or a little lower, repre- 

 sents the ectodermal substance, and in it is located the material for the ciliated 

 plates. At the lower pole and in the central part of the egg is the material sub- 

 stratum of the endoderm. In all other well established cases the ectodermal sub- 

 stances lie near the animal pole, while the mesodermal and endodermal substances 

 lie near the vegetal pole. Inasmuch as an apical sense organ is formed at the 

 animal pole in ctenophores in much the same way as in annelids, mollusks and 

 nemerteans, it is difficult to resist the conclusion that the localization of the ecto- 

 dermal, mesodermal and endodermal substances in the ctenophore egg will ulti- 

 mately be found to be similar to that which prevails in other types. 



3. Echitiodcrm Type. 



The form of localization in the eohinoderm egg, as shown by Boveri's 

 (1901) work on Strongylocentrotits, is in many respects similar to the annelid- 

 mollusk-nemertean type. In this case, however, the mesoplasm is located at the 

 lower pole of the egg and is sorrounded hy an equatorial zone of endoplasm, whereas 

 in annelids and mollusks, after the first two cleavages, the endoplasm lies at the 

 lower pole, and the mesoplasm on the posterior side of this pole, and in one only of 



