MUSEUM OF COMPARATIVE ZOOLOGY. 329 



the plane of segmentation. These, however, are not always the first 

 to be withdrawn. The " vacuoles " may result from a mechanical 

 stimulus inaugurated by the rupture of the nucleus. (!) 



As segmentation advances, the cells become successively smaller, and 

 the nuclei, though absolutely smaller, are proportionately much larger 

 than at first. They grow at the expense of the yolk, which ultimately is 

 entirely consumed, — a confirmation of the theory that the germinative 

 vesicle is the primary egg cell. 



The influence of abnormal fecundation on the process of cell division 

 as given by Fol ('77*) will be found in the part of the present paper 

 devoted to fecundation. 



McCrady ('77) seeks the explanation of segmentation in the amoeboid 

 nature of his *' protembryo " (p. 177), and is compelled to assume the 

 existence of an unseen protoplasmic matrix to explain, for example, the 

 approach and mutual flattening of segmentation spheres after their 

 separation. He finds himself compelled to admit that 0. Hertwig's ob- 

 servations make it impossible to exclude a selective polar force, and he 

 proceeds to show how this polar force may be supposed to operate in 

 segmentation. Although apparently irreconcilable, McCrady thinks 

 the two theories (Hertwig's and his own) may not be really incompatible. 



Brandt ('77*) has communicated observations on the eggs of Lymnaeus 

 stagnalis and Anodonta anatina, in which the amoeboid nature of the 

 germinative vesicle and of the nucleus is deemed sufficient to account for 

 the phenomena accompanying maturation and division. 



In the case of Lymnseus (pp. 591 - 593) the condition of the nucleus 

 during the first part of the first segmentation was not satisfactorily fol- 

 lowed, but during the stage of the mutual flattening of the segments the 

 nuclei were seen. At this time they appeared as clear structures of a 

 stellate shape, which were possessed of ramified, more or less radial pseu- 

 dopodia, and were constantly undergoing changes of form. It was owing 

 to the instability of their forms that they were at times very distinct 

 and sharply defined ; at other times less distinct, or even invisible. 



After citing several authors whose observations are thought to be 

 capable of an interpretation substantiating the amoeboid hypothesis, the 

 author says (p. 598): ''Im Ganzen herrscht bei den citirten Autoren 

 die Ansicht vor, die strahlenfdrmigen Figuren waren auf ein Structur- 

 verhaltniss des Dotters zu beziehen. Meine friiheren Beobachtungen 

 am Ascaridenei zwangen mich dieser Ansicht entgegenzutreten und die 

 Strahlen fiir Pseudopodien der Furchungskerne zu halten; zwischen 

 ihnen miissen eo ipso auch die Dotterkornchen sich strahlenfdrmig 



