36 EMBRYOLOGY. 



of the germinative vesicle and the origin of the egg-nucleus preceded fertilisa- 

 tion, and thus I distinguished between the phenomena of maturation and 

 fertilisation of the egg-cell, which generally had been interchanged and con- 

 founded. I also endeavoured to make it probable that the egg-nucleus 

 descended from the germinative vesicle, and in fact from a nucleolus of the 

 vesicle, and defended the thesis that the egg during its maturation did not 

 pass through a non-nuclear condition. In this I fell into an error : I overlooked, 

 like all previous observers, the connection between the formation of the polar 

 cells and the disappearance of the germinative vesicle, a process which it was 

 the more difficult to establish in the object which I studied because it takes 

 place in the ovary. 



The excellent investigations of BUTSCHLI, which brought the changes of the 

 germinative vesicle into connection with the formation of the polar cells, now 

 made their appearance, supplementing my results. The polar cells were 

 discovered in the year 1848 by FR. MULLER and LOVEN, and were named by 

 the former directive vesicles (Richtungsblaschen), because they always lie at 

 the place where subsequently the first cleavage-furrow makes its appearance. 

 Their wide distribution in the animal kingdom had also been established by 

 many investigators ; BUTSCHLI was the first, however, to direct attention to 

 the peculiar processes which take place in the yolk, in the interpretation of 

 which he, nevertheless, committed several errors. He maintained that the 

 whole germinative vesicle is converted into a spindle-shaped nucleus, which 

 moves to the surface, and, while becoming constricted in the middle, is thrust 

 outside by the contractions of the yolk in the form of two directive bodies. 

 By this process the egg became non-nuclear, and again acquired a nucleus 

 only in consequence of fertilisation. 



In two further articles on the Formation, Fertilisation, and Cleavage of the 

 Animal-Egg, I modified the teachings of BUTSCHLI, and brought them into 

 unison with my previous investigations, inasmuch as I pointed out that 

 the germinative vesicle is not as such directly converted into the nuclear 

 spindle, but in part is dissolved : that the spindle takes its origin from the 

 nuclear substance in a manner which it is very difficult to investigate ; that 

 the polar cells are formed, not by the elimination of the spindle, but by a 

 genuine process of division or budding ; that in consequence of this the egg is 

 not destitute of a nucleus even after the constricting off of the second polar 

 cell, but that the egg-nucleus arises from the half of the divided polar spindle 

 which remains in the yolk, and therefore, in its ultimate derivation, from 

 components of the germinative vesicle of the immature egg. 



Soon afterwards BUTSCHLI also interpreted the development of the directive 

 bodies as cell-budding, likewise GIARD and also FOL, who has produced a 

 very extensive and thorough investigation on the phenomena of the maturation 

 of the egg in animals. Kecently VAN BENEDEN, supported by researches on 

 Nematodes, has conibatted the interpretation of the process as cell-budding; 

 however, BOVERI and 0. ZACHARIAS, who have established a complete agreement 

 between the formation of directive bodies and the process of cell-division in 

 the case of the Nematodes also, are unable to subscribe to his conclusion in 

 this matter. 



As a new advance is to be recorded the discovery by WEISMANN and by 

 BLOCHMANN, that in eggs which are developed parthenogenetically only a 

 single polar cell arises. 



If the original obscurity on the morphological side, in which the phenomena 



