64 



BIOLOGICAL LECTURES. 



Conklin (4) was the first to call attention to the possible 

 importance of the sphere-substance in the cleavage of the Q.^%. 

 He summarizes his results on the Qg^ of Crepidiila thus : "After 

 the first two cleavages the sphere-substance is differently dis- 

 tributed to the different cells, the entire sphere-substance of 

 one generation always going into those cells of the next genera- 

 tion which lie nearest the animal pole. This differential distri- 

 bution of the spheres has been followed through every cleavage 

 up to the twenty-four-cell stage. As the form of the cleavage 



is perfectly constant, it 

 follows that the sphere- 

 substance of any genera- 

 tion goes into certain 

 definite cells which have a 

 perfectly constant origin 

 and destiny. This differ- 

 ential distribution of the 

 spheres is not caused by 

 ,, , , ^ , . r , their specific weight, since 



1"IG. 20. — Unio. r irst cleavage ; the elongation of the ^ o ' 



sphere-substance in the larger cell and of the cell their mOVemeUtS arC thc 

 itself marks the plane of the second cleavage. In a . , . . 



slightly later stage there is a well-marked constriction SamC m WhateVCr pOSltlOU 



across the cell in the position of the future second thg cq\\ maV be nlaCPd Tt 



cleavage-spindle in this cell. 



seems to be the result of 

 a form of polarity which, like that of the ^gg itself, is not 

 the result of gravity. 



"The centrosomes do not, apparently, arise from the sphere- 

 substance of the previous division, but some distance from it, 

 and the sphere-substance never divides, but each sphere ulti- 

 mately grows ragged at its periphery and gradually fades out 

 into the general cytoplasm. 



"The differential distribution of the spheres and their 

 subsequent conversion into cytoplasm suggest that they 

 may be important factors in the differentiation of cleav- 

 age cells, and if further investigation should establish the 

 fact that they are, in part, composed of the oxychro- 

 matin of the nucleus, it would furnish a basis in fact for 

 certain well-known speculations of de Vries, Weismann, 

 and Roux." 



