226 THE CONTINUITY OF THE GERM-PLASM AS THE 



^upport to the explanation of Minot and Balfour, for in all cases 

 in which polar bodies have not been found in parthenogenetic eggs, 

 these structures are also absent from the eggs which require fertiliza- 

 tion in the same species. But although the expulsion of polar 

 bodies in parthenogenesis has not yet been proved to occur, we must 

 assume it to be nearly certain that the phenomena of maturation, 

 whether connected or unconnected with the expulsion of polar 

 bodies, are the same in the eggs which develope parthenogenetically 

 and in those which are capable of fertilization, in one and the same 

 species. This conclusion depends, above all, upon the phenomena 

 of reproduction in bees, in which, as a matter of fact, the same egg 

 may be fertilized or may develope parthenogenetically, as I shall 

 have occasion to describe in greater detail at a later period. 



Hence when we see that the eggs of many animals are capable of 

 developing without fertilization, while in other animals such de- 

 velopment is impossible, the difference between the two kinds of 

 eggs must rest upon something more than the mode of transforma- 

 tion of the nucleus of the germ-cell into the first segmentation 

 nucleus. There are, indeed, facts which distinctly point to the con- 

 clusion that the difference is based upon quantitative and not 

 qualitative relations. A large number of insects are exceptionally 

 reproduced by the parthenogenetic method, e. g. in Lepidoptera. 

 Such development does not take place in all the eggs laid by 

 an unfertilized female, but only in part, and generally a small 

 fraction of the whole, while the rest die. But among the latter 

 there are some which enter upon embryonic development without 

 being able to complete it, and the stage at which development 

 may cease also varies. It is also known that the eggs of higher 

 animals may pass through the first stages of segmentation without 

 having been fertilized. This was shown to be the case in the egg 

 of the frog by Leuckart l , in that of the fowl by Oellacher 2 , and 

 even in the egg of mammals by Hensen 3 . 



Hence in such cases it is not the impulse to development, but the 



1 R. Leuckart, article ' Zeugung,' in R. Wagner's ' Handwbrterbuch der Phy- 

 siologic,' 1853, Bd. IV. p. 958. Similar observations were made by Max Schultze. 

 These observations appear however to be erroneous, for PflOger has since shown that 

 the eggs of frogs never develope if the necessary precautions are taken to prevent the 

 access of any spermatozoa to the water. A. W., 1888. 



a Oellacher, ' Die Veriinderungen des unbefruchteten Reims dea Hiihncheneies. 

 ' Zeitschrift fiir wissenschaftliche Zoologie,' Bd. XXII. p. 181. 1873. 



3 Hensen, ' Centralblatt,' 1869, No. 26. 



