THE EARLY DEVELOPMENT OF THE CHICK 247 



cells, their nuclei continue to divide, while accompanying 

 cytoplasmic divisions are 

 either incomplete or en- 

 tirely lacking. A large 

 number of free nuclei is 

 thus formed in the margin 

 of the blastodisc. These 

 nuclei, continuing to mul- 

 tiply, wander out into the 

 marginal periblast, con- 

 verting this into a nu- 

 cleated though non-cellu- 

 lar tissue. From this re- 

 gion some of the nuclei 

 migrate inward below the 

 blastodisc, converting the 

 central periblast also into 

 a nucleated structure, ex- 

 cept in its middle area, 

 above the nucleus of 

 Pander, which appears to 

 remain free from nuclei. 

 The nucleated rim of the 

 periblast forms a part of 

 what is known later as the 

 germ wall. 



The circular blastoderm 

 now begins to extend radi- 

 ally, partly on account of 

 the growth of its own cells, 

 and partly (continuing 

 Blount's account) by the 

 addition of cells from the 

 marginal periblast. The 

 region of the original blas- 

 todisc now becomes thin- 

 ner and more transparent, and is known as the area pellucida, 



