384 HOLMES. [Vol. XVI. 



1 believe, for holding the accounts of the formation of four or 

 more generations of ectomeres to be erroneous. 



The last author who records more than three generations of 

 ectomeres is Fujita, who studied the cleavage of the pulmonate 

 Siphonaria. After describing the formation of the four-cell 

 stage, he says : " During the next following stages four succes- 

 sive generations of micromeres are budded off from each of 

 the above-mentioned segments, now to be called macromeres. 

 Hereupon the macromere D is entitled to the name of ento- 

 mesoderm, and the remaining three macromeres may be called 

 entodermic macromeres. Synchronously with the formation 

 of the fourth generation of micromeres, each member of the 

 third generation divides, thus giving rise to a fifth generation. 

 At this stage there are twenty micromeres and four macro- 

 meres, the relations of which may be seen in Fig. 2. Next 

 comes in order the formation of a sixth generation of micro- 

 meres again from the third, followed by that of a seventh 

 from the fifth." It seems to me quite certain that Fujita over- 

 looked the division of the first generation of micromeres and 

 concluded that the four outer cells resulting from this cleav- 

 age arose from the macromeres. This is a very natural and 

 easy error to make, as I can testify from experience, having 

 been deceived, for a time, on just this point, when working on 

 another form. Fujita describes no division of the first quar- 

 tette until after the stage in which the egg contains thirty-four 

 cells, when the cells of the second quartette have divided twice, 

 and the posterior cells of the third have divided. Moreover, 

 Fujita's figure of the twenty-four-cell stage shows that it corre- 

 sponds exactly, so far as the relations of the cells are concerned, 

 with the same stage in Planorbis, Physa, and Crepidula. If we 

 assume that the cells marked 2 in Fujita's Fig. 3 are the tro- 

 choblasts, the genealogy of all the cells would exactly corre- 

 spond to that in the above forms, and the following divisions 

 up to the forty-third-cell stage, which Fujita describes, would 

 correspond point for point with those of Planorbis, The cells 



2 are smaller than the apical cells, and their position indicates 

 that they arose by a laeotropic division, as would be expected 

 according to the principle of alternation of spirals. All of these 



