No. 3-] PODARKE OBSCURA VERRILL. 415 



the 64-cell stage, the appearance of the cell X1.2, and the 

 bilaterally symmetrical cross form a series of orientation 

 marks which are unmistakable. We may say, therefore, that 

 in A and B quadrants the first division of the cross is very 

 unequal ; in C and D quadrants it is nearly equal. 



TJie Appearance of Bilaterality . 



This marking out of a bilateral plane by cleavages which 

 follow a true radial ^ type is important to my mind as indicat- 

 ing that the bilateral divisions have nothing to do with shift- 

 ings of material into the median plane, but rather that the 

 bilateral and the radial are distinct in origin. The former 

 having been, so to speak, superimposed upon the latter, the 

 bilaterality of the organism may express itself even in the 

 spiral cleavages- 

 Bilateral cleavages have not in Podarke such direct refer- 

 ence to the form of the body, as distinct from the head, as 

 Conklin (No. 5, a) has described for Crepidula. The first bilat- 

 eral cleavage is at the lower pole (PL XXXVIII, Fig. 27), but 

 bilateral divisions appear early and are very prominent at the 

 upper pole as well. Further, as we have seen, bilateral sym- 

 metry is established before bilateral cleavages appear, and is as 

 marked at the upper as the lower pole. 



Of interest in this connection are the following observations : 

 I have seen a few (not more than three were carefully studied) 

 cases where the cross, when formed, is radial and not bilateral. 

 Each arm has three equal cells, like the anterior cross arms 

 represented in PI. XXXVII, Fig, 18. In these same embryos 

 2di and 2d2 divided just like the other second quartette cells, 

 and no small X1.2 appeared. In this stage, therefore, the 

 embryo was still radially symmetrical. In one other case three 

 cross arms were alike in having three equal cells, while the fourth 

 was of the posterior cross-arm type. These facts are signifi- 

 cant to my mind as indicating a reversion to a radial type of 

 a cleavage which had secondarily become bilateral. The fact 



1 In this and later discussions I have followed Conklin in classing both "ortho- 

 radial " and " spiral " cleavages under the general head of " radial." 



