THE EGG-CELL OR OVUM. I05 



the pole, alters considerably in its structure, and divides. A 

 minute cell, with half of the nucleus, and a small amount of 

 l^rotoplasm, is given off. Not long after, the nucleus remaining 

 within the ovum repeats the 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 general, 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 con- 

 siderable diversity as to the exact time at which the extrusion 

 occurs ; generally, however, it precedes the entrance of the 

 fertilising 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 

 spermatozoon has been observed to penetrate them. Usually, 

 however, they simply dwindle away. The remaining female 

 nucleus of the ovum is now ready to unite with the male 

 nucleus of the spermatozoon. 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 fertilisation, 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 the eggs, which only developed after fertilisation, two 

 occurred as usual. To these facts we must afterwards recur 

 in connection with parthenogenesis. 



