66 



PROTOPLASMIC CONSISTENCY AND CELL DIVISION 



and hence the egg must elongate. After elongation the surface of 

 the egg seems to tear in the plane separating the two semisolid spheres. 

 The periphery of the two asters of the amphiaster stage never becomes 

 so firm as their interior. This may account for the observation of 

 von Erlanger/" confirmed by Spek/^ who described peripheral currents 

 in the rapidly dividing nematode egg. In this egg peripheral currents 

 flow from the two poles toward the equator and from there inward 

 to the center of the egg. Spek suggests that such currents exist in all 

 dividing eggs, and that they are easily visible in the nematode egg 

 because of the great rapidity with which it segments. ConkHn^^ de- 

 scribed an inward flow of granules at the equator of the dividing Crepid- 



FiG. 12. Change in shape of an Asterius ovum (c) before and (b) after completion 

 of the first cleavage furrow. 



ula egg, and I^ have observed a similar current, although a very slow 

 one, in the sand-dollar egg. 



Immediately after cleavage both of the two blastomeres are more 

 or less spherical; but later, when they become more fluid, they are 

 pressed against each other so as to be flattened at the plane of contact. 



^^ von Erlanger, R., Beobachtungen iiber die Befruchtung und erstenTeilungen 

 an den lebenden Eiern kleiner Nematoden, Biol. Centr., 1897, xvii, 152, 339, 



^^ Spek, J., Oberflachenspannungsdifferenzen als eine Ursache der Zellteilung, 

 Arch. Entwcklngsmechn. Organ., 1918, xliv, 5. 



^"- Conklin, E. G., Protoplasmic movement as a factor of differentiation, Marine 

 Biol. Lab., Biol. Led., 1899, 69. 



