7 i 4 THE GENERATIVE ORGANS. 



the other yelk-cells. Henking further claims to have discovered 

 the rudiment of a germinal vesicle in ripe eggs and to have 

 traced the first segmentation spindles. As the latter could 

 only occur after impregnation, the important question in rela- 

 tion to the views I have expressed is, whether a germinal 

 vesicle really exists in the unripe ovarian eggs, and on this 

 point Henking's results appear to me to be no more satisfactory 

 than those of his predecessors. I cannot admit that there is 

 any evidence that his germinal vesicle differs from the nuclei 

 of the other yelk-cells. 



The changes which occur in the germinal vesicle previously 

 to fertilisation are now very well known in many Invertebrates, 

 Echinoderms, Ascaris, etc., and have also been observed in 

 some Vertebrates, as in the Rabbit, so that there is no doubt 

 these phenomena are of very general and probably universal 

 occurrence. 



Either before, or immediately after, the ovum is discharged 

 from the ovary it undergoes changes preparatory to, but 

 entirely independent of, fertilisation. The germinal vesicle 

 approaches the surface of the vitellus, loses its distinctness of 

 outline and after exhibiting karyokinetic changes which are 

 characteristic of a nucleus about to divide, first one and then 

 a second portion separates and is extruded from the yelk. The 

 two minute bodies which are extruded from the yelk are known 

 as polar bodies or directive corpuscles. The remainder of 

 the nucleus, which remains within the yelk, is termed the 

 female pronucleus. 



The Polar Bodies or Directive Corpuscles are not to be con- 

 founded with the polar globules of Weismann. They are 

 developed before, and not after, impregnation. Weisrnann's polar 

 globules are nucleated cells, and their appearance is contempo- 

 raneous with the formation of the blastoderm. They are pre- 

 cisely similar to segmentation spheres. It is unfortunate that 

 two structures so entirely different should have received similar 

 names, but the term polar globule was used by Weismann 

 before directive corpuscles were known. 



The Formation of Directive Corpuscles in Insects. Robin [360] 



