SUMMARY OF CURRENT RESEARCHES 



RELATING TO 



ZOOLOGY AND BOTANY 



(principally invertebrata and cryptogamia), 



MICEOSCOPY, Etc.* 



ZOOLOGY. 



.VERTEBRATA. 

 a Embryolog-y.f 



Natural and Artificial Parthenogenesis.^ — A. Petrunkevitck takes 

 a retrospect of recent work and discussion on parthenogenesis, aud seeks 

 to show how the question now stands. Our knowledge of fertilisation 

 and parthenogenesis may be recapitulated in four statements. 



1. Both the egg ready for fertilisation and the mature sperm show a 

 reduction in the number of chromosomes of their nuclei to one half of 

 that found in somatic cells. 



2. No matter how many spermatozoa succeed in entering the egg, 

 the nucleus of only one of them, under normal conditions, fuses with 

 the egg nucleus, thus restoring the original number of chromosomes. 

 All other spermatozoa are absorbed. 



h\ The centrosome of the egg disappears after the second polar cell 

 is formed, its functions being assumed by the centrosome of the sperma- 

 tozoon. 



4. In most parthenogenetic eggs no reduction of chromosomes takes 

 place — only one polar cell being formed — and the egg centrosome 

 remains active. 



The spermatozoon may introduce substances apart from its chromo- 

 somes and its centrosome ; it may be necessary (with Bethe and 

 Bresslau) to distinguish between Besamung and BefrucMung, but it 

 seems certain that the paternal hereditary characters are transmitted to 

 the descendant by the chromosomes of the sperm-nucleus, and that the 

 sperm-centrosome gives the stimulus to development and controls the 

 successive divisions. 



* The Society are not intended to be denoted by the editorial " we," and they 

 do not hold themselves responsible for the views of the authors of the papers noted, 

 nor for any claim to novelty or otherwise made by them. The object of this part of 

 the Journal is to present a summary of the papers as actually published, and to 

 describe and illustrate Instruments, Apparatus, etc., which are either new or have 

 not been previously described in this couutry. 



t This Section includes not only papers relating to Embryology properly so called, 

 but also those dealing with Evolution, Development, Reproduction, aud allied subjects. 



X Amer. Nat., xxxix. (1905) pp. 65-76. 



