No. 3.] STUDIES ON CEFHALOPODS. 259 



into future ectoderm and entoderm. And again, the formation 

 of a certain organ, or 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, as is shown by the well-known researches 

 of Whitman on Clepsine, or more recently in the admirable work 

 of Wilson on Ltcinbricus and Nereis. The more carefully the 

 phenomena are studied, the more astonishing is the regularity 

 and precision with which the cleavage process is conducted and 

 the differentiation of tissues is accomplished. 



The occurrence of variations or irregularities in the mode of 

 cleavage in a certain animal — irregularities as judged by the 

 arrangement of superficial cytoplasmic furrows — does not in- 

 validate the importance of the conclusion which can be derived 

 from the study of forms where a great regularity prevails. For 

 the essential 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 cleav- 

 age, or whether it be divided into dissimilar portions and the 

 equilibrium of growth be gradually secured during the subse- 

 quent stages of cleavage. The distribution of the nuclear sub- 

 stance 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 instance, as my 

 friend Dr. C. Ishikawa tells me, the summer and winter eggs 

 of a certain form of Daphnidae undergo different " types " of 

 cleavage, one being holoblastic and the other being meroblastic, 

 the difference being probably produced by the amount of food- 

 yolk. Dr. Ishikawa has kindly placed at my disposal several 

 interesting drawings which illustBate this point clearly. The 

 accompanying figures show the difference of the modes of 

 cleavage between the yolkless summer egg and the winter egg, 

 in which food-yolk is abundant, _/ww one and the same species of 

 animal. The summer egg belongs to the regular, holoblastic 

 type of cleavage, and the winter egg, to the meroblastic type, 

 showing a close resemblance to the ova of some insects. 



