590 



HAROLD HEATH, 



to the fact that when the tij) cells are formed each is in contact with 

 the trochoblasts of the first quartette of adjacent quadrants, and con- 

 sequently the groups of cells which are to enter into the formation 

 of the prototroch form a ring about the embryo broken at one point 

 on its posterior side. During their subsequent division these groups 

 maintain essentially the same position, and when every cell that is to 

 enter into the formation of the velum is present the relations are 

 practically unchanged. But in the functional condition the prototrochal 



Pig. B. Successive stages in the develop- 

 ment of the prototroch. Fijjs. a, b and c 

 represent changes underj^one in quadrnnts A, 

 B and C. In d and e the posterior quadrant 

 is represented. The post - trochal cells are 

 stippled ; the somatic plate {Sp) is shaded and 

 stippled. The cells a;, x' aid in the formation 

 of the velum and are derived from the median 

 cell of the cross. 



cells form two rows encircling the embryo, while in diagram B or 

 Fig. 27 the prototroch is two cells wide as regards the first quartette 

 trochoblasts but only one row wide in the second (tip cells). The 

 method by which the completed condition arises is most interesting 

 and affords a phase of cell dynamics that is not easily explained by 

 simple mechanical principles. 



In diagram B a, it will be seen that a cell of the present lower 

 row tal-^-'^'-^ is in contact with the right tip cell 'Ja^^K Sub- 

 sequently it belongs to the upper row (c, diagram B) while la'^ '-^ l 

 and id'-^--^ will come to lie in contact with the right and left tip 

 cells respectively which are situated in the lower row. The movements 

 are not brought about by a simple shifting of the cells in question 



