( 



48 THE DEVELOPMENT OF THE CHICK 



is no way of timing the development, so that one has to judge 

 the sequence of the stages, all of which come from the uterus, by 

 the degree of formation of the shell, by the size of the cells and 

 b}" the appearance of the sections. This can be at best only 

 approximate; and, as the securing of any given stage is largely 

 a matter of chance, no one has, as a matter of fact, secured a 

 complete series. In the pigeon, on the other hand, the time 

 since laying the first egg is a fairly exact criterion of the stage 

 of development of the second egg. It has, therefore, been pos- 

 sible to secure a complete series, and the subject has been worked 

 out by Miss Blount, whose preliminary communication in Vol. 

 XIII of the Biological Bulletin furnishes the basis of the following 

 account. 



The periblast ring is entirel}^ devoid of nuclei after the super- 

 numerary sperm-nuclei have degenerated. The marginal cells 

 become greatly reduced in size owing to multiplication and 

 continuous production of central cells, and their nuclei thus 

 approach more and more closely to the periblastic ring. The 

 scene then changes; the marginal cells cease to produce central 

 cells; when their nuclei divide the peripheral daughter-nuclei 

 move out into the periblast, which is thus converted into a nu- 

 cleated syncytium. The periblastic nuclei multiply rapidly and 

 invade all portions of the periblastic ring, which maintains its 

 original connection with the white yolk. Not only do the peri- 

 blastic nuclei invade the periblastic ring, but some of them also 

 migrate centrally into the protoplasm forming the floor of the 

 segmentation cavity. They do not, however, reach the center, 

 but leave a non-nucleated sub-germinal area, corresponding 

 approximately to the nucleus of Pander, free from nuclei. The 

 subgerminal syncytium may be known as the central periblast 

 to distinguish it from the marginal periblast. They are, of 

 course, continuous. In sections one has the appearance of nuclei 

 in the yolk, for there is no sharp boundary between periblast 

 and yolk (Fig. 22). The syncytium, which has received its nuclei 

 from the marginal cells, is the primordium of the germ-wall (Figs. 

 21, 22, 23, 24). 



There is a snarp contrast between the segmented blastoderm 

 and the syncytial periblast not only in structure Init also as 

 regards fate. The marginal cells constitute a zone of junction be- 

 tween blastoderm and periblast. Thus in Fig. 22 it will be ob- 



