292 WATASE. [Vol. IV. 



enon of fusion. It may have been an imperfect division at first, 

 however. At any rate, the behavior of these four corresponding 

 segments on each side of the median axis is interesting. The 

 specimen as it now stands has lost all the faint cleavage lines 

 which existed at first, and the four blastomeres have completely 

 fused with each other. The origin of this mass of fused seg- 

 ments can be traced to a single blastomere at the eight cell 

 stage. In the diagram. Fig. XIX, the segments «./.' and a.r.^ 

 on both sides of the first cleavage furrow (I'-i) represent the 

 original segments which gave rise to the four imperfectly divided 

 segments described above. 



Another curious example which I have met is one in which 

 the abnormal caryokinesis in one segment on one side of the 

 body is shared by the corresponding segment on the other (/./.' 

 and/,/'.', Fig. XVIII). In both segments mentioned above, the 

 nuclei were seen in the state of being divided into three nuclei, 

 as indicated by the triasters. 



The fact that the groups of cells which vary simultaneously 

 on both sides of the bilateral ovum can be traced back to a 

 single segment in an earlier stage appears to me to be a matter 

 of considerable importance. Although I was unable to trace the 

 whole history of such segment or segments, it is probable they 

 give rise to or constitute a corresponding part of some future 

 bilateral structure. Another fact, namely, the unusual mode of 

 caryokinesis occurring in the corresponding segments of the 

 bilateral blastoderm when all other cells in the body are dividing 

 normally, is a point worthy of consideration. 



And finally, we must bear in mind that the precocious cleav- 

 age may occur in the descendants of a single blastomere alone 

 and not in pairs, as in Fig. 24. 



This phenomenon of imequal growth of different parts, as I 

 may call it in an empirical way, has been observed by several 

 naturalists. 



Brooks ^ figured unequal cleavage in the bilateral blastoderm 

 of Batrachus. 



His 2 developed this fact of unequal growth of different parts 

 into a principle, although his principle more especially refers 



^ Alter nation of Periods of Rest with Periods of Activity in the Segmenting Eggs of 

 Vertebrates. Studies from Biol. Laborat., Johns Hopkins Univ., Vol. II, 1882. 

 2 Unsere Korperform, etc., 1874. 



