118 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Figures 95-121 (Plates 11, 12) show how close is the resemblance 

 to the cleavage of L. aiiatifera. Except in size and some unimportant 

 details, the various stages of the two species are indistinguishable, and 

 the description of the figures of L. anatifcra may be applied to those of 

 L. fascicularis. 



A renewed study of the few old preparations, supplemented by many 

 new ones, shows that I ('96) was wrong in the conclusion that the 

 ectoblast is detached from the yolk-macromere by means of four succes- 

 sive divisions ('96, cctomeres A, B, C, and D). The supposed fourth ecto- 

 mere ('96, Figs. G and 7 D) is the primary mesoblast cell. In origin and 

 position it corresponds exactly with the mesoblast cell (d^-^) seen in the 

 sixteen-cell stage of L. anatifera. I now interpret the spindle seen in 

 the yolk during the fifth cleavage ('96, Fig. 7), which was then supposed 

 to represent the separation of the mesoblast and the entoblast, as a 

 rare case of precocious division of the entoblast. Study of the complete 

 series, with all mitotic phases represented, shows that in L. fascicularis, 

 as in L. anatifera, the first, second, and third cleavages form micro- 

 meres containing the ectoblast and " secondary mesoblast," while the 

 fourth cleavage separates mesoblast and entoblast from each other. 



With regard to the planes of cleavage and orientation, I find no 

 important disagreement with L. anatifera. The descriptions of the 

 first and second cleavages in the preliminary note were similar to those 

 of L. anatifera given in this paper. The rotation during the first 

 cleavage was not then known. The equatorial nature of the third 

 cleavage was not clearly shown by the figure of a four-cell stage with 

 inclined spindles in the preliminary note ; Figures 100-103 (Plate 

 11) in this pai)cr better represent the four-cell stage and the third 

 cleavage. The figure of the eight-cell stage ('96, Fig. 6) was drawn 

 from an egg which is now .known to have been incorrectly oriented. 

 Eggs which give exactly such camera tracings will, when properly 

 oriented by moving the cover glass, always show the same arrangement 

 of cells as that seen in Figures 104-lOG in this paper. 



Figure 6 of the preliminary paper represented a separation of 

 mesoblast and entoblast (fourth cleavage), and not as was incorrectly 

 assumed, the formation of a "fourth ectomere." Figures' 108-110 are 

 the corresponding figures in this paper. 



Tlie primary mesoblast cell, shown in Figure 8 of the preliminary 

 paper as filling the blastopore, represented the delayed fifth cleavage, 

 which was in progress. The single entoblast nucleus was not yet under- 

 going the fifth cleavage. The inferred connection between the spindle 



