246 OUTLINES OF CHORDATE DEVELOPMENT 



retain their connection with one another peripherally, in the 

 deeper region of the marginal cells. 



We should note here a few details regarding the history of the 

 accessory or supernumerary spermatozoa. During the period 

 intervening between fertilization and the early cleayages, these 

 form nuclei which migrate to the outlying portion of the blasto- 

 disc. There they may divide once or twice, forming small 

 groups of daughter nuclei. These divisions are sometimes 

 accompanied by slight indications of cytoplasmic division, and 

 the short superficial grooves thus formed are known as the 

 accessory cleavages (Fig. 89). These are visible during the four- 

 and eight-cell stages; they are usually radial in direction, lying 

 just across, or without, the margin of the blastodisc. No true 

 cells are formed by such cleavages. The accessory sperm 

 nuclei degenerate rapidly, the accessory cleavages fade out, 

 and by the time thirty-two cells are formed no traces of these 

 structures are left. 



During subsequent cleavages the number of central cells 

 increases rapidly, by additions from the dividing marginal cells, 

 and through their own multiplication, which becomes very 

 rapid as the cells diminish in size. Additional horizontal cleav- 

 ages appear in the central cells, converting the roof of the 

 blastocoel into a layer several cells in thickness (Fig. 90, (7). 

 No cells are added to the germ disc from the floor of the 

 segmentation cavity. The marginal cells become greatly 

 shortened through the continued cutting off of central cells, 

 and finally they are limited to the extreme margin of the 

 blastodisc. 



About the time two or three hundred cells are formed, inter- 

 cellular furrows extend out into the marginal periblast (Fig. 89, 

 E). Up to this time both central and marginal periblast have 

 been entirely free from nuclei, but soon these areas, which are 

 still directly continuous, become converted into a nucleated 

 syncytium. The details of this process have not been described 

 for the chick, but are well known in the pigeon (Blount). In 

 the latter form, when the marginal cells have become reduced 

 to an approximately spherical form by the cutting off of central 



