96 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



tions of this sort are those of Robin (1875) and Whitman (1878) on the eggs of 

 leeches. Here peculiar aggregations of protoplasm occur at the two poles of the egg, 

 after maturation and fertilization, which have been called " polar rings." Vejdovsky 

 (1888) discovered that these polar rings arise in Rhynchelmis from a peripheral 

 layer of brown protoplasm, which has a great affinity for stains. The substance of 

 this layer collects at the two poles of the egg after maturation and fertilization and 

 thus constitutes the polar rings. During the cleavage most of this substance is seg- 

 regated into the large posterior macromere of the 4-cell stage, and it ultimately 

 passes into the mesomeres (probably the first and second somatoblasts of Wilson). 

 Nevertheless other portions of this protoplasm go into the micromeres; in fact it 

 forms " the general material for the building of the body, with the exception of the 

 intestinal epithelium" (Vejdovsky. 1888, p. 123). Polar rings have also been 

 observed by Foot (1S04. 1896) in AUolobophora, and their method of formation in 

 this form has been determined in a most careful and satisfactory manner ; this work 

 will be discussed more fully in the next section on the genesis of egg organization. 



One of the most remarkable cases on record of the localization of visibly differ- 

 ent kinds of ooplasm is found in Myzostoma glabrum in which Driesch (1S!J0). and 

 more recently Carazzi (1004), observed two conspicuous zones of protoplasm in the 

 egg before maturation, an upper one which is of a redish tint and a lower one which 

 is green. During the maturation of the egg the upper zone differentiates into 

 two. an upper red zone and an equatorial colorless one. According to Driesch 

 (1896. p. 120) the red zone gives rise principally to the substance of the micro- 

 meres (ectoderm), the clear zone to endoderm. and the green one to the substance 

 of the somatoblasts (ectoderm and mesoderm). 



Another case of visible localization of the substances of the unsegmented egg 

 was observed by Boveri ( L901) in the ovocyte, egg and larva of Strongylocentroius 

 lividus ; here before maturation and fertilization the surface of the egg is covered 

 by a uniformly distributed red pigment; after maturation this gathers into an equa- 

 torial zone leaving an area of clear protoplasm at the upper pole and another at 

 the lower one. Later development shows that the upper clear cap gives rise to the 

 ectoderm, the red zone to endoderm and the lower cap to mesenchyme. 



A visible localization of differently colored substances in the unsegmented egg- 

 also occurs in fresh water snails belonging to the genera Physa, Planorbis and 

 Limncza. In these animals I have found (Conklin. 1903) that a clear cap of pro- 

 toplasm appears at the upper pole during maturation and then gradually spreads, 

 over the upper hemisphere of the egg; the upper hemisphere thus becomes milky- 

 white in the living; egg, while the lower half remains yellow. 1 have followed these 



O 00- 7 J 



white and yellow substances through the development and find that the white sub- 

 stance gives rise to the ectoderm, the yellow to the mesoderm and endoderm. 



Quite recently Wilson ( L904) has observed in Dentalium a localization of unlike 

 substances in the unsegmented egg and by a series of experiments he has shown 

 the part which some of these substances take in the formation of certain organs of 

 the larva. As in the case of Strongylocentroius there is here an accumulation of 



