50 Pub. Puget Sound Biol. Sta. Vol. 2 No. 34 



tion such that a cleavage plane passing perpendicularly to it would cut 

 off a inicromere in the position in which it would come to rest at this stage. 

 That this might be the case seems probable if we consider the rotation as 

 due to a seeking of equilibrium of position. Viguier (1898) takes this 

 point of view concerning both the cause of rotation and the cause of the 

 typical alternation of cleavages. He considers the spiral inclination of 

 the nuclear spindles, however, to be the result of an intensification by 

 heredity of the movements caused by the equilibrating forces. He says: 



"On pent admettre que, primitivement, cette rotation dcs micromeres 

 ne s'effectuait qu'apres la fin de la division. Devenue hereditaire, elle 

 commence d'une fa^on plus precoce, ainsi que le montre le changement de 

 direction de fuseau nucleaires ; mais elle ne peut cependant s'achiver que 

 lorsques la division est complete. . . . Me parait tres comprehensible en ad- 

 mettant que tant cela resulte des changements de position deter- 

 mines par les lois de I'equilibre, et qui, par heredite acceleree, apparaissent 

 avant la separation complete des cellules." 



A cross-furrow may appear in the upper quartet, but it is inconstant. 

 The micromeres contain none of the dense yolk material and are almost 

 clear when mounted ; whereas the macromeres are dense with deutoplasm. 

 Because there is a markedly smaller amount of yolk present in the eggs of 

 Haminea than in those of Crepidula the contrast in the size of the macro- 

 meres and micromeres is less marked than in Crepidula, the situation 

 being essentially the same as in Tethys and Umbrella. 



FOURTH CLEAVAGE 



The fourth cleavage consists of the laeotropic division of both quartets 

 synchronously, giving rise to 16 cells, 12 micromeres and 4 macromeres 

 {Fig. 16). From the upper or first micromere quartet arise four smaller 

 cells (la^-ld") which Conklin has termed the turret cells. The four re- 

 maining micromeres of the first quartet are now termed la'-ld^ in accord- 

 ance with Conklin's nomenclature. From the four macromeres arise the 

 second quartet of micromeres (2a-2d), smaller than their parent macro- 

 meres but larger than the micromeres of the first quartet {Fig. 10). 



In Nereis the first quartet divides once at the time the second is being 

 formed, and before the third is formed it divides again. In Crepidula, 

 Tethys, Planorbis, and Fiona it divides once before the third is formed, 

 ;ind in Umbrella it does not divide at all before the formation of the third. 

 Of this Conklin (1897) says: "In general, the rate of development of 

 the upper hemisphere is indicated by these facts ; in Nereis the develop- 

 ment of the upper hemisphere is very precocious ; it is very tardy in 

 Umbrella; while Crepidula occupies an intermediate position in this re- 



