BILATERALITY OF THE PIGEON's EGG 297 



represent vacuoles in life, shows a finer meshwork about the 

 germinal vesicle than elsewhere. These smaller vacuoles sur- 

 round the germinal vesicle (g. v.) symmetrically on all sides, 

 except where it adjoins the peripheral protoplasm and from this 

 area the blastodisc is differentiated. The same conditions may 

 be observed in figure 27 in which, however, the germinal vesicle 

 has been distorted by compression in sectioning. The develop- 

 ment of the blastodisc symmetrically about the germinal vesicle 

 and the coincident changes within the latter indicate that 

 the former differentiates in close association with the germinal 

 vesicle and lend support to the suggestion made above that the 

 nuclear axis of bilaterality is transmitted to the blastodisc. In 

 figure 27 the deeply staining yolk granules, which are present 

 throughout the cell except in the peripheral protoplasm and the 

 latebra, are small in the neighborhood of the germinal vesicle 

 (g. V. ) ; they remain as the characteristic granules of the blasto- 

 disc. Throughout development they remain connected with the 

 typical white-yolk spheres by all possible transitional forms and 

 are to ,be looked upon simply as deutoplasm in a form that can 

 be immediately assimilated by the protoplasm. This 'digested' 

 yolk persists only about the germinal vesicle, characterizing the 

 blastodisc. Fig. 8 is from an oocyte with a clearly defined anlage 

 of the blastodisc surrounding the germinal vesicle, which in Hfe 

 has the form of a biconvex lens. Centrally, and at the peri- 

 pheral margin, the blastodisc merges with the bed of white yolk, 

 the intermediate zone being the anlage of the central and marginal 

 periblast of Blount ('09), the finely granular region about the 

 germinal vesicle giving rise to the segmental disc (see figs. 35, 40 

 and 43 for the regions in the mature egg). By this time the 

 peripheral protoplasm has become narrower, less so over the 

 blastodisc than over the rest of the periphery (fig. 29). 



Oocytes at the end of the period of differentiation, i.e., in the 

 resting stage before the final rapid growth, have the blastodisc 

 well developed; all the definitive ooplasmic structures are laid 

 down. Fig. 7 shows the blastodisc of such an oocyte in oblique 

 surface view, from which aspect the germinal vesicle appears 

 circular. A median but obhque section of the same oocyte (fig. 



