No. I.] THE EMBRYOLOGY OF CREPIDULA. 43 



the egg is not bilaterally symmetrical with reference to the first 

 cleavage plane, but it is radially symmetrical ; the blastomeres 

 are congruent antimeres, and the egg at this stage is a "one- 

 rayed radiate," as Chun ('80) calls the Ctenophores. The 

 radial symmetry of the egg prevails undisturbed from the time 

 polarity is first established (p. 39) until the primary mesoblast 

 is formed (p. 67). After this event the posterior half of the 

 egg becomes more or less bilateral, while the anterior half re- 

 mains radially symmetrical. Finally, at a relatively late stage 

 the entire egg becomes bilateral. 



The rotation of blastomeres in some of the later stages of 

 cleavage has long been known and commented upon. So far 

 as I can find Selenka ('81) first used the term spiral in this 

 connection. He described in the polyclades a " laeotropen 

 Oder /V-Spirale " in the formation of the first quartette of micro- 

 meres, and a "dexiotropen oder S-Spirale " in the formation of 

 the second quartette, but he did not apply either of these terms 

 to the earlier or later cleavages. Lang ('84) first called atten- 

 tion to the fact that the second cleavage in Discocoelis takes 

 place in a " left-wound spiral." Since then this same fact has 

 been observed in the case of many other animals (cf. Conklin 

 ('91), Wilson ('92), Heymons ('93), Lillie ('95), et al.), and, with 

 one or two exceptions which will be described later, the direc- 

 tion of this cleavage is invariably the same. 



Up to the present, however, no one has shown that the fiist 

 cleavage also is a spiral one. In all other works on this sub- 

 ject, so far as I am aware, it is asserted that the position of 

 the spindles during the second cleavage is the first indication 

 of spiral cleavages (see Wilson ('92), pp. 387, 453, Heymons 

 ('93), p. 249, Lillie ('95), pp. 14, 15). 



I believe, however, it may be safely asserted that in all 

 cases in which the second cleavage is laeotropic the first is 

 dexiotropic, and that the initial cause of the spiral cleavages is 

 not to be found in the direction of the nuclear spindles, but 

 rather in the structure of the unsegmented egg itself. 



