448 EMBRYOLOGY OF THE CHICK 



the underlying undivided protoplasm, leaving a little space, which is the 

 beginning of the segmentation cavity or blastocoele (Figs. 257, II B, and 

 258). The undivided protoplasm beneath is called the central periblast, 

 the original periblastic region being now known as the marginal periblast. 

 Both of the periblastic regions retain their connection with each other 

 peripherally in the deeper regions of the marginal cells. 



The question that may arise here is, "What has become of the ac- 

 cessory or supernumerary spermatozoa?" Between the time of fertili- 

 zation and the first cleavage, these have formed nuclei which migrated 

 to the outlying portion of the blastodisc. There they probably divided 

 once or twice to form small groups of daughter nuclei. There even 

 seems to be an attempt of the cytoplasm to divide, and sometimes short 

 superficial growths are actually formed. These are called accessory 

 cleavages. They can be seen during the four and eight cell stage, usually 

 radial in direction, lying just across or outside the margin of the blasto- 

 disc. No true cells, however, are formed by such cleavages. The ac- 

 cessory sperm nuclei all degenerate rather rapidly, the accessory cleav- 

 ages fading away, so that by the time the embryo has reached the thirty- 

 two cell stage, no traces of these accessory structures can be found at all. 

 As cleavage continues, the number of central cells increases very 

 rapidly, by the marginal cells dividing and being added to the central 

 cells, although the central cells divide likewise. This latter multiplica- 

 tion is very rapid, the cells diminishing in size. For example, cleavages 

 appear in the central cells, causing the roof of the blastocoele to become 

 several cells in thickness. There are no cells added to the germ disc 

 from the floor of the segmentation cavity. The continual cutting off of 

 central cells from the marginal cells causes these latter to be consid- 

 erably shortened, until finally they are limited to the extreme margin of 

 the blastodisc only. 



After division has taken place so that two or three hundred cells 

 have been formed, there are intercellular furrows extending out into the 

 marginal periblast. Up to this time, there have been no nuclei what- 

 ever in either central or marginal periblast, but two areas which are 

 continuous now become converted into a nucleated syncytium. Our 

 knowledge of this developing process comes from the study of the 

 pigeon. It has not been worked out in the chick. The process is some- 

 what like this: The marginal cells have become spherical in form, by 

 having the central cells cut off from them. Their nuclei now divide, 

 although the cytoplasmic divisions are either completely lacking or do 

 not completely divide. The free nuclei therefore become quite extensive 

 in the margin of the blastodisc, and as these nuclei continue multiplying, 

 they wander off into the marginal periblast so that nuclei are scattered 

 about quite thickly, though the structure itself is non-cellular. Some 

 of the nuclei also migrate inward below the blastodisc, so that the cen- 

 tral periblast is likewise converted into a nucleated structure with the 



