No. I.] THE EMBRYOLOGY OF CREPIDULA. I 77 



cleavage. It will not do, therefore, to lay too great emphasis 

 on the alternation of cleavages. It must be confessed that 

 the expression spiral cleavage is open to objection, since in 

 many cases no real spiral is formed either by the spindles or 

 the cell walls. Still, if taken apart from its colloquial mean- 

 ing, the word spiral clearly and specifically designates a partic- 

 ular kind of cleavage which needs a distinctive and technical 

 name, and it may be doubted whether any other name would 

 not be open to as serious criticism. 



On the other hand, Wilson's exposition of the spiral type of 

 cleavage is in certain places open to objection. He says ('92, 

 p. 378): "The cell divisions . . . show a peculiar modification of 

 radial symmetry, which is best characterized as spiral in char- 

 acter, and which cannot be reduced to the bilateral type." The 

 suggestion, here contained, that there is a third kind of sym- 

 metry, viz., spiral symmetry, is still further borne out by the 

 three coordinate types of cleavage which he establishes, viz., 

 the radial, the spiral, and the bilateral. That such a classifica- 

 tion, either of symmetry or of cleavage, is unjustifiable is 

 shown, I think, by the fact that so far from being " a peculiar 

 modification of radial symmetry," the spiral symmetry, thus 

 suggested, is one of the most common forms of radial symmetry, 

 and likewise the " spiral type of cleavage " is by all odds the 

 most common representative of radial cleavage. Spiral cleav- 

 ages, therefore, belong entirely to the radial type, and should 

 not be classified as coordinate with either the radial or bilateral 

 types. 



I shall limit the use of the term spiral to those cases in 

 which cleavage occurs in the same direction in each quadrant, 

 i.e., it is always a purely radial cleavage. If this radial char- 

 acter is changed even in one out of four quadrants, it would 

 then be better to use the term oblique. Oblique cleavages 

 then might be or might not be bilateral, but they would not 

 be radial. As we shall see later, oblique cleavages, using the 

 term in this special sense, are transitional between spiral and 

 bilateral cleavages. 



One of the most constant and characteristic features of all 

 radial cleavage is the alternation of direction in successive 



