1/2 MARINE BIOLOGICAL LABORATORY. 



manner that one of the new nuclei by its division gives 

 rise to the right, and the other to the left side of the 

 adult organism. In another case, it has been main- 

 tained, the first division of the nucleus distributes the 

 nuclear substance into future ectoderm and entoderm. 

 And again, the formation of a certain organ, or of a 

 system of tissues, has been traced in a most definite 

 manner to a particular cell or group of cells in an early 

 stage of cleavage. The more carefully the phenomena 

 are studied, the more astonishing is the regularity and 

 the precision with which the cleavage process is con- 

 ducted and the differentiation of tissues is accompHshed. 



The occurrence of variations or irregularities in the 

 mode of cleavage in a certain animal — irregularities 

 as judged by the arrangement of the superficial cyto- 

 plasmic furrows — does not invalidate the importance of 

 the conclusion which can be derived from the study of 

 forms where absolute regularity prevails. For the es- 

 sential feature of the cleavage process is the division 

 and distribution of the nuclear substance of the ovum, 

 and in so far as the nuclear substance is distributed in 

 such a manner as to produce a symmetry of growth in 

 the developing organism, it is immaterial whether its 

 total quantity be divided exactly in two equal halves 

 and distributed into right and left at the first cleavage, 

 or whether it be divided into dissimilar portions and the 

 equilibrium of growth be gradually secured during the 

 subsequent stages of cleavage. The distribution of 

 the nuclear substance may have been just as accurate 

 and precise. in one case as in the other. 



A comparative study of cleavage of different ova 

 affords another example illustrating this point. For 



