THE EGG-CELL OR OVUM. 



99 



process, and another tiny cell is expelled. This process, which the 

 majority of investigators regard as one of normal cell-division or- cell- 

 budding, is known as the extrusion of the polar globules. Of gen- 

 eral, and probably of universal occurrence, it has been but rarely 

 observed in fishes and amphibians, and not as yet demonstrated in 

 reptiles or birds. It was for long thought to be absent in arthropods, 

 but the researches of Weismann, Blochmann, and others, have shown 

 that this is not the case. An interesting peculiarity, which we shall 

 afterwards notice, has been demonstrated by Weismann in regard to 

 parthenogenetic ova. There is considerable diversity as to the exact 

 time at which the extrusion occurs; generally, however, it precedes 

 the entrance of the fertilizing sperm. The minute extruded cells never 

 have any history, though they occasionally linger for a considerable 

 time on the outskirts of the ovum. As an exception, they have been 

 seen themselves to divide; and, with equal rarity, a misguided sper- 

 matozoan has been observed to penetrate them. Usually, however, 

 they simply dwindle away. The remaining female nucleus of the 

 ovum is now ready to unite w :th the male nucleus of the spermatozoan. 

 By the twofold division just described it has been considerably reduced 

 in size, though not a whit in complexity, or in the number of its 

 chromatin elements. At this point, awaiting the essential moment of 

 fertilization, we shall for the present leave it. 



Within the last two years Weismann, assisted by C. Ischikawa, 

 has demonstrated an exceedingly interesting fact in regard to polar 

 globule extrusion in parthenogenetic ova. Instead of the two polar 

 globules which are usually extruded, parthenogenetic ova were shown 

 to form only one. This was demonstrated in a variety of cases, — in 

 water-fleas (daphnids and ostracodes) and rotifers, — and is believed 

 by this eminent authority to be a general fact. Blochmann, who has 

 been successful in demonstrating polar globules in several orders of 

 insects, has also observed that in the parthenogenetic ova of the plant- 

 louse or aphis, only one polar globule was formed, while in the eggs, 

 which only developed after fertilization, two occurred as usual. To 

 these facts we must afterwards recur in connection with par- 

 thenogenesis. 



IX. Theories of the Polar Globules. — The polar globules appear to have 

 been first observed in 1848 by Fr. Miiller and Lov£n, but it is only within recent 

 years that much has been made of them. Thanks to the masterly researches of 

 Biitschli and Hertwig, Giard, Fol, and others, it became possible to interpret 

 the extrusion as a case of cell-division or budding. More recently, Van 

 Beneden, whose monograph on the ovum of the threadworm (Ascaris) will 

 remain one of the classics in this department of research, has raised a protest 

 against regarding the extrusion as a normal cell-division. The details of the 



