6oo WILSON. [Vol. VIII. 



I. In the bilateral type (Diagram C) the cell-divisions are 

 symmetrical with respect to a single plane — usually that of the 

 first cleavage — which generally coincides with the median 

 plane of the adult body. This appears in astonishing perfection 

 in the tunicate [Clavelina, Van Beneden and Julin, No. 31) and 

 in the cephalopod {Loligo, Watase, No. 32) in both of which 

 the bilaterality persists up to a relatively late stage and con- 

 forms to the symmetry of the adult. It occurs also in the 

 ctenophore (Agassiz i, Chun 6), in many bryozoa (Barrois, 2) 

 in AmpJiioxiis and in some vertebrates. 



II. In the radial type (Diagram A) there are two systems 

 of cleavage-planes of which one set are meridional, and radially 

 symmetrical to the egg-axis, while the other set intersect 

 the meridians at right angles — i.e., they pass through the 

 equator or are parallel to it. This form occurs in the sponge 

 Sycandra (Schultze, No. 27) and in great perfection in some 

 echinoderms (Antedoft, Seeliger, No. 28, Echinus, Selenka, 

 No. 30). 



III. The spiral type (Diagram B) arises from the radial 

 through a twisting of the radii, as it were, the blastomeres 

 being displaced or rotated, with respect to the egg-axis, either 

 to the right, following the hands of a watch (right-handed 

 spiral) or in the reverse direction (left-handed spiral) as the 

 case may be. (The direction of rotation typically alternates in 

 successive cleavages; cf. Nereis, No. 35.) The term "spiral," 

 refers to the fact that the curved radii, if prolonged, would form 

 a spiral about the egg-axis.^ 



In this case the two sets of cleavage planes are both oblique 

 to the egg-axis, though approximately rectangular to each other. 

 The spiral type occurs in great perfection throughout the 

 annelids, mollusks ^excepting cephalopods) and polyclades, and 

 it is a remarkable fact, that throughout all these forms the 

 direction of rotation is constant. In all, namely, the first group 

 of micromeres (formed at the third cleavage) is rotated to the 



^ The term spiral was first applied to this form of cleavage as far as I am aware 

 by Selenka {29) in the case of Polyclades, and was afterwards adopted by Lang 

 (18). (Barrois No. 3 had already used it, in a different sense, in the case of 

 nemerteans.) It is perhaps not very happily chosen, and the term alternating or 

 alternate type might be preferable. 



