580 HAROLD HEATH, 



a distinct bulging of the upper right hand side of the cells occurs 

 before the division is completed. When the blastomeres are formed 

 they will be seen to lie some distance to the right of the parent cells ^), 

 and therefore partly over the furrows between the macroraeres. This 

 shifting occurs principally before the complete division of the cells, 

 although in many cases the central spindle is slightly bent showing 

 that a shifting has occurred after the cells are formed. But this 

 latter stage of the rotation is almost imperceptible, and is not unlike 

 numerous examples which may be noted throughout embryonic 

 development. 



2. Formation of Primary Trochoblasts and the Second 

 Quartette of Ectomeres, 16 cells (Figs. 6—9), 



In accordance with the law of the alternation of cleavages as 

 formulated by Kofoid ('94) this stage is introduced by a leiotropic 

 division of the first quartette which forms four cells of equal size, 

 termed by Wilson the trochoblasts. Usually at the same time 

 the macromeres cleave in a leiotropic direction forming the second 

 quartette of ectomeres. The spindles giving rise to the trochoblasts 

 and to the second quartette are almost in line with each other, and 

 consequently when the cells are completely formed they lie pressed 

 together between the macromeres on one side and the stem cell of 

 the first quartette on the other. 



Since the advent of Professor Wilson's beautiful study of Nereis 

 in which for the first time the origin of the prototroch was accurately 

 determined, a number of papers have appeared which greatly advance 

 our knowledge of this organ in other forms. Among Annelids there 

 is Amphitrite and Clymenella (Mead), Podarke (Treadwell), Areni- 

 cola (Child), and in Gastropods Crepidula (Conklin), in which the 

 accurate development of the prototroch is known. And also there 

 are many forms, including Annelids, Gastropods, Polyclades and Chitons, 

 in which there is an essential similarity of origin and behavior of like 

 cells in the early stages. As their later history has not been deter- 

 mined nothing definite may be claimed, yet it appears altogether 

 probable that in this respect they do not differ from the above ac- 

 curately studied forms. At all events this fact remains, that among 



1) In the tables of cleavage I have rigidly adhered to the rules 

 given on page 576, but throughout the descriptions I have used the 

 term parent or stem cell in a looser sense, designating it as the larger 

 of the two products of a cleavage. 



