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



It seems to me further probable that the orientation of his 

 figure 1 2a is incorrect ; it will be noticed that the cross-furrow 

 between E2 and Ei, is at right angles to its direction in \\A. 

 It is easier to believe that Rabl has mistaken one side of the 

 embryo for the anterior or posterior end. This might easily 

 be done ; for the macromeres are equal in size and the only 

 means of orienting them is the cross-furrow, which is invisible 

 from the apical pole. Thus the apical cross-furrow in 1 2 A is 

 probably at right angles to the vegetative as in 11^4, and not 

 parallel as represented. This table also shows fairly accurately 

 the cleavage of Limax according to Kofoid. 



In the table of cleavages of Umbrella (Table III) I have not in- 

 cluded all of the details described by Heymons after the forty- 

 cell stage. Heymons has followed the cleavage cell by cell up to 

 about 100 cells. The table given does not as a consequence 

 give a correct impression of the immense detail of Heymons' 

 work. The stages described after the 40-cell stage are : the 

 44-, 47-, 51-. 55-, 57-, 63-, ^1-. 69-, 75-, 81-, 91-cell stages. The 

 cleavage of the entomeres was followed far beyond these stages. 

 It is interesting to notice the almost purely arithmetical pro- 

 gression in the increase in number of the cells after the four- 

 cell stage. The disturbances in the regularity of this law are 

 due to precocious separation of important blastomeres. E.g.y 

 24 to 25 cells due to formation of the mesoblast (z/. table) ; 

 37 to 38 due to bilateral cleavage of mesoblast ; 38 to 40 

 another bilateral cleavage, separating excretory cells E and 

 E^\ 55 to 57 bilateral divisions of Mesoblasts. A better illus- 

 tration of Rabl's too-inclusive law could not be desired. The 

 cleavage in Unio {v. table, p. 33) shows that precocious segre- 

 gations may entirely destroy the orderly progression. The 

 same table up to the twenty-five-cell stage at least will do 

 equally well for Crepidula. Conklin has followed the cleavage 

 very much farther, but has not yet published the details m 

 such a way that they can be tabulated. 



As Wilson has pointed out, it is probable that v. Wisting- 

 hausen (Table VI) has overlooked one cleavage of the posterior 

 macromere so that the mesoblast would arise from the fourth, 

 not from the third cleavage of this cell. 



