CLEAVAGE AND DIFFERENTIATION. 37 



general, the cells formed are similar in position and size, viz., 

 there are four macromeres, three quartettes of micromeres, and 

 the first quartette is surrounded by a belt composed of the 

 second and third quartettes. The first quartette undergoes 

 three spiral divisions in alternate directions, and the second 

 quartette divides once. Here the resemblance with the poly- 

 clade ceases, though the annelid and gasteropod go one step 

 further in these likenesses, viz., (3) the three quartettes of 

 micromeres are ectomeres in the annelid and gasteropod, and 

 (4) in both these groups the mesoblast is formed from the cell 

 4-d, which gives rise to paired mesoblastic bands. 



" Beyond this point Wilson believed that the annelid diverged 

 from the gasteropod. He supposed that the * cross ' in the two 

 was wholly different both in origin, position, and destiny, and 

 that the velum had a wholly different origin from the annelidan 

 prototroch. 



" Lillie has extended all the above-mentioned resemblances 

 between annelids and gasteropods to the lamellibranchs, and in 

 addition has discovered the following : (5) the first somatoblast 

 (2d), which gives rise to the ectoderm of the trunk, has exactly 

 the same origin and position and a similar history in the anne- 

 lid and lamellibranch ; (6) it gives rise to a growing point and a 

 ventral plate in all respects essentially like those of the annelids. 

 Lillie shows good reason for believing that in other mollusks 

 the posterior growing point is derived from these cells. 



" To this list of resemblances between the annelid and the 

 mollusk, which I can confirm in the case of the gasteropod, I 

 have been able to add the following : (7) the rosette series of the 

 gasteropod is exactly like the cross of the annelid in origin, 

 position, and probably in destiny. The intermediate girdle cells 

 of the annelid are like the cross of the gasteropod in origin, 

 position, and destiny (at least in part) (Fig. 12). The differ- 

 ences, therefore, between the annelidan and molluscan cross 

 which Wilson emphasizes are not real ones; (8) the trocho- 

 blasts of the annelids and gasteropods are precisely similar in 

 origin and destiny (at least in part) (Figs. 10, 12). In some 

 annelids (Amphitrite, Clymenella, Arenicola), the prototroch is 

 completed by cells of the same origin as in Crepidula and Neri- 



