6 KAEYOKINESIS. 



problems of heredity and diflferentiation are matters of common knowledge and need 

 not be discussed here ; the relation of cleavage to these lai-ger problems is neither so 

 generally i^ecognized nor so freely admitted. 



In the cleavage of the egg, differentiations occur to a remarkable degree in 

 certain cell divisions, while they appear to be absent in others. Typically, cell 

 divisions are rythmical, alternating, qualitatively and quantitatively equal, and con- 

 sequently non-differential. The differentiations of cleavage cells are due to depart- 

 ures from this typical condition in one or moi-e particulars. In certain animals 

 these departures are very notable, the cleavage being from the first differential. The 

 differentiations of cleavage may have a far-reaching prospective significance, since 

 in certain animals (annelids, mollusks, polyclades and nematodes) the principal axes 

 and body regions of the future animal are marked out by the cleavage planes and 

 entire organs are represented by a single cell or group of cells. In such cases the 

 minutest details of unequal, bilateral or qualitatively dissimilar division of cells may 

 be of great importance. The forms and peculiarities of such cleavage are inherited 

 quite as certainly as are any adult features, and when the jiroblem of inheritance 

 may be reduced to a certain peculiarity of a certain cell division it is evident that 

 we have this problem reduced to relatively simple terms. 



The species which has formed the chief subject of this work and from which 

 all the figures are drawn is Crepidula plana Say. The following species and genera 

 have also been studied more or less completely : — 



C. formcata Lam., C. convexa Say. C. adunca Keep, UrosalpiJix cinerea Stimp- 

 son, Sycotypus canaliculatus Gill, Fulgur carica Conrad, Haminea solitaria Say, 

 y^olis papulosa Loven. 



Many of the phenomena here recorded, particularly those relating to proto- 

 plasmic movement and the history of the centrosomes during cleavage, have been 

 observed in all of these seven species of Prosobranchs and two species of Opistho- 

 branchs. These phenomena are, therefore, not isolated, and it is probable that they 

 are of wide occurrence. 



Methods. 



The eggs of Crepidula are in many respects peculiarly favorable for study. These mollusks are 

 very abundant and the eggs are deposited in large numbers, a single female usually laying from eight to 

 ten thousand eggs. No other egg with which I am acquainted is so favorable for the study of cleavage. 

 The eggs are inclosed in membranous capsules, which reagents readily penetrate ; however, in order to 

 insure rapid fixation I have usually punctured the capsules with a needle before putting them into the 

 fixing fluid ; they are then passed through the various reagents and finally imbedded and sectioned 

 while still in the capsules. Each female of C. plana deposits an average of fifty capsules, with approxi- 

 mately 175 eggs in each capsule. The eggs vary scarcely at all in size, each being about 0.136 mm. in 

 diameter. The great advantage of being able, without further trouble, to handle in large numbers such 

 small eggs will be at once apparent. On the other hand the eggs are unfavorable in having a relatively 

 large cjuantity of yolk which is colored deeply by most stains. 



Two general methods of observation have been followed : (1) the study of entire eggs ; (2) the 

 study of serial sections. For the former the best method of preparation is as follows : The living eggs 

 are teased from the capsules inco Kleinenberg's picro-sulphuric fluid, or into Bovei-i's picro acetic, 

 where they are left from thirty minutes to two hours; they are then washed in alcohol until the eggs 

 become nearly white, and are stained in the following solution for from five to ten minutes. 



