No. I.] THE EMBRYOLOGY OF CREPIDULA. 193 



believed that mollusks had only the gastrula form in common 

 with other animals. Wolfson ('80) and Fol ('76), who main- 

 tained that there were agreements between the early cleavage 

 stages in gasteropods and lamellibranchs, were opposed by Rabl 

 ('79), Hatschek ('80), and Blochmann ('81), who held that there- 

 were no detailed resemblances. Blochmann concluded that the 

 cleavage in Chiton does not belong to the gasteropod type ; 

 and although he pointed out several resemblances between the 

 cleavage of gasteropods and of turbellarians, no one supposed 

 that outside the molluscan phylum any exact or long-continued 

 resemblances to molluscan cleavage would be found. I recall 

 with what astonishment Professor Wilson and the writer found, 

 only a few years ago, that the cleavage of Nereis and Crepidula 

 was so wonderfully similar in many respects. Wilson ('92) 

 called attention to many of these resemblances, though at that 

 time I think he did not suspect that they were as numerous 

 nor as precise as they have since been found to be. Lillie's 

 ('95) work added some very important points of resemblance 

 between the cleavage stages of the annelids and the mollusks, 

 and in this work I have been able to add still others. 



Wilson ('92) emphasizes the following important resemblances 

 between the early cleavage stages of the annelid, the polyclade, 

 and the gasteropod: (i) the number and direction of the cleav- 

 ages is the same in all three up to the 28-cell stage; (2) in 

 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, 



