500 STANLEY C. BALL 



to a pair of cells between which there is a marked disparity 

 in size. Text figure 10, a reconstruction of the two embryos 

 of one capsule, shows very well the relative proportion in size 

 of the first micromere to the macromere. It also indicates that 

 gravity may not be sufficient to determine from which pole of 

 the egg the micromere shall arise, for in this capsule the polarity 

 of one egg is exactly opposed to that of the other. Not too 

 much reliance, however, is to be placed upon the polarity as 

 indicated in sectioned worms. When the animals are dropped 

 into the killing fluid the tissues are as likely as not to become 

 fixed while the specimens are inverted. Hence if the embryos 

 rotate with respect to gra\dty, their positions with respect to the 

 mother might easily be changed while the fixing fiuid was pene- 

 trating her body-wall and parenchA^ma. But since both embryos 

 are equally free to rotate, the evidence afforded by text figure 

 10 suggests that gravity does not wholly determine the embryo's 

 polarity. Indeed, this figure, taken in conjunction with figures 

 14 and 18, indicates that the opposed polarity invariably 

 exhibited by the two embryos of a capsule in later stages is 

 already determined at the time of the first cleavage of the egg. 



According to Patterson's account the first cleavage of the 

 fertilized egg of P. gemellipara results in the splitting off of a 

 micromere of such a size that it is necessary to count the chromo- 

 somes in order to distinguish it from the first polar body. We 

 find, however, (text fig. 12), that this micromere is at least 

 twice as large as the polar body, and furthermore (text fig. 10) 

 that its nucleus immediately enters a resting stage during which 

 it may be more or less amoeboid. No such nucleus has been 

 found in the polar bodies. Owing to the rapidity with which 

 the latter disintegrate, it is often impossible to find both the 

 first polar body and micromere present together so as to com- 

 pare them directly. 



It is characteristic of the blastomeres of Paravortex gemelli- 

 para, as of P. cardii and other Turbellaria, that the nuclei are 

 strikingly vesiculated. 



Hallez asserts that in P. cardii the first micromere divides 

 before the fu'st cleavage of the macromere. Text figure 10 



