ROBERT CHAMBERS 65 



the amphiaster (Fig. lie), forming about it, resulted in a second un- 

 equal cleavage with the formation of Blastomere /3 (Fig. 11 d). The 

 projecting piece of the egg above the obliterated furrow remained 

 quiescent during these divisions and not until after the third unequal 

 cleavage resulting in the formation of Blastomere 7 in Fig. lie, did 

 it become incorporated in Blastomere 8. 



In this experiment the cut was probably made in the egg when the 

 process for the first cleavage was too far advanced for the egg to 

 retrace its course. The gash was therefore not obliterated and a 

 very peculiar condition resulted in a succession of advances of the 

 cleavage process about the gash. Blastomere a, being the earliest 

 formed, segmented ahead of its fellows (Fig. 11 d). Blastomere /3 

 came next (Fig. 11 e). Unfortunately before Blastomeres y and 5 

 divided the egg died. 



It is significant that Blastomere 5 is larger than 7 as evidently the 

 former finally incorporated the hitherto inactive part of the egg that 

 lay above that part of the original first cleavage furrow which lay on 

 the right side of the gash (Fig. 116). 



///. Concerning the Mechanism of Cell Division. 



The changes in shape that an echinoderm egg undergoes during 

 cleavage can be in part understood on the assumption that the astral 

 formation is a solidifying process. It has long been known that at 

 the time of cleavage the eggs of echinoderms, many worms, mammals, 

 etc., become elongated,^ the cleavage furrow forming in a plane at 

 right angles to the long axis of the egg. As the furrow deepens, each 

 resulting blastomere tends to assume the shape of a sphere (Fig. 12 a). 



Nobody, however, has thus far been able to explain the cause of 

 this elongation. The observations recorded in this paper may ex- 

 plain this phenomenon. The two spheres of soHdification grow at 

 the expense of all but possibly a small peripheral part of the fluid 

 egg substance. The combined diameters of the two fully formed 

 semisolid spheres are greater than the original diameter of the egg, 



^Hertwig, 0., Beitrage zur Kenntniss der Bildung,Befruchtungund Theilung 

 des thierischen Eies, Morph. Jahrb., 1876, i, 347. Gurwitsch, A., Morphologic 

 und Biologie der Zelle, Jena, 1904. 



