I06 CONK LIN. [Vol. XIII. 



differs from the other three, and correspondingly we find it 

 gives rise to parts of the embryo wholly unlike those which 

 arise from either of the other arms. 



The significance of the cross, therefore, as indeed of all the 

 most important features of the cleavage, is prospective ; its 

 cause is to be sought in some peculiarity of protoplasmic struc- 

 ture rather than in any extrinsic mechanical factors. 



2. The Turret Cells {Trochoblasts). 



The turret cells were formed by the first division of the first 

 quartette of micromeres. Until the tip cells of the cross are 

 formed they are much the smallest cells in the entire &^^. 

 Gradually, however, they increase in size, until they become 

 much the largest cells of the egg, excepting the yolk cells. 

 This remarkable increase in size is not due to the fact that 

 they grow so much more rapidly than other cells, for this they 

 do not do, but to the fact that their growth is continuous, and 

 not interrupted by any divisions until a very late stage. 



The turret cells lie in the angles between the arms of the cross. 

 Until a late stage they are in contact with the apical cells from 

 which they sprung ; but with the longitudinal splitting of the 

 arms of the cross and the formation of the rosette series they are 

 pushed away from the apical cells, though they continue to lie in 

 the angles between the arms. The two posterior turrets hold 

 this position as long as they can be recognized at all. The an- 

 terior ones are crowded farther and farther outward and down- 

 ward by the cells derived from the anterior and transverse arms. 



I have never seen the turret cells in process of division, but 

 believe that the anterior ones divide at about the stage shown 

 in Figs. 49, 50, Diagrams 9 and 10 ; in the earlier figures the 

 division has not taken place, in the later ones it has. The pos- 

 terior turret cells divide very seldom, if at all. They remain 

 very large, much larger than the anterior ones, and lie on each 

 side in the angle between the anterior and posterior branches 

 of the velum ; they ultimately assist in forming the walls of 

 the head vesicle. Certain large cells adjoining the posterior 

 turrets appear to have come from the latter by division, but I 

 do not know that this is true. 



