348 ON THE NUMBER OF POLAR BODIES AND 



its expulsion. In parthenogenetic eggs two polar bodies are also 

 formed in most cases, but they arise from the subsequent division 

 of the single body which separates from the egg. But such sub- 

 sequent division is only of secondary importance as far as the egg 

 itself is concerned, and is also unimportant in the interpretation of 

 the process. The essential nature of the process is to be found in 

 the fact that the nucleus of the egg-cell only divides once when 

 parthenogenesis occurs, but twice when fertilization is necessary, and 

 it is of no importance whether the expelled part of the nucleus of 

 the cell-body atrophies at once, or after it has undergone division. 

 We have, therefore, to distinguish between primary and secondary 

 polar bodies. If this distinction is recognized, and if we leave out 

 of consideration all doubtful cases mentioned in literature, such a 

 large number of well- established observations remain, that the 

 existence of two primary polar bodies in sexual eggs, and neither 

 a smaller nor a larger number, may be considered as proved. 



Hence follows a conclusion which I believe to be very significant, 

 the difference between parthenogenetic and sexual eggs lies in 

 the fact that in the former only one primary polar body is ex- 

 pelled, while two are expelled from the latter. When, in July, 

 1886, I published a short note 1 on part of the observations made 

 upon parthenogenetic eggs, I confined myself to facts, and did not 

 mention this conclusion. I took this course simply because I did 

 not wish to bring it forward until I had made sufficient observa- 

 tions in the first of the two ways described above. I had hoped 

 to be able to offer all the proofs that can be obtained before 

 undertaking to publish the far-reaching consequences which would 

 result from the above-mentioned conclusion. Unfortunately the 

 material with which I had hoped to quickly settle the matter, 

 proved less favourable than I had expected. Many hundred 

 sections through freshly laid winter-eggs of Bythotrephes longi- 

 manns were made in vain ; they did not yield the wished for 

 evidence, and although continued investigation of other material 

 has led to better results, the proofs are not yet entirely com- 

 plete. 



I should not therefore even now have brought forward the above- 

 mentioned conclusion, if another observer had not alluded to this 



1 Weismann, ' Richtungskorper bei parthenogenetischen Eieren,' Zool. Anzeiger, 

 1886, p. 570. 



