82 CONK LIN. [Vol. XI 1 1. 



has been said, this cell, which is lacking at this stage in the 

 other species, is present, and the ectoblast is radially symmetri- 

 cal. The four macromeres may still be considered radially 

 symmetrical. The cells of the fourth quartette lie on the sides 

 of the egg in the furrows between the four macromeres, but 

 the radial symmetry of the egg is destroyed by the behavior of 

 the posterior member of this quartette, 4d. All the divisions 

 of this cell except the first one are purely bilateral in position. 



IV. History of the First Quartette of Ectomeres. 

 Diagrams 6-ii. 



Owing to the presence of certain peculiar landmarks, I have 

 been able to follow the cell lineage of the first quartette 

 farther than that of the others. The derivatives of this first 

 quartette give rise to the whole apical region of the embryo, 

 viz., all the ectodermal cells of the head vesicle, an apical 

 plate of ciliated cells, the posterior cell plate, the dorsal portion 

 of the functional velum and a portion of the first velar row on 

 the ventral side, the supraoesophageal ganglia and commissure, 

 the cerebro-pedal connectives, and, possibly, the pedal ganglia, 

 an apical sense organ, and the paired eyes. 



Wilson ('92) has shown that in Nereis limbata and N. megalops 

 this quartette gives rise not only to the entire upper hemisphere 

 of the trochophore, but also to the head kidneys and all the 

 cells of the prototroch. This is not the case in Crepidula ; all 

 that portion of the velum which lies at the ends of the right, 

 the left, and the anterior arms of the cross being derived from 

 the second quartette, while only the intermediate portions come 

 from the first quartette. The anterior branch of the velum on 

 the dorsal side of the body is also derived from the first quar- 

 tette. As to the head kidney,^ it is not present in marine 

 prosobranchs, as is well known. 



The first division of the cells composing this first quartette 

 gives rise to the turret cells, as has been described (p. 58). 

 Their second division occurs immediately after the formation of 

 the mesentoblast 4d, as shown in Fig. 23 ; each of the four 



^ "Wilson has lately suggested that this structure may be a mucous gland as 

 Mead found to be the case in Amphitrite. 



