MATURATION OP THE EGG, AND PROCESS OF FERTILISATION. 35 



Although the researches on the phenomena of maturation of 

 the egg in animals still present numerous gaps, nevertheless it 

 can be regarded as already well-established, that eggs with a germi- 

 native vesicle are never capable of fertilisation, that the germinative 

 vesicle is without exception dissolved, and that there is formed out of 

 components of it (as regards the details there are still many processes 

 to be more carefully studied) a very small egg-nucleus. During the 

 metamorphosis there arise, probably ivithout exception, polar cells. 



The polar differentiation of many eggs rich in yolk, which was 

 pointed out in the first chapter, may be brought into causal connection 

 with the phenomena of maturation. Without exception the animal 

 pole is the part of the egg-sphere to which the germinative vesicle 

 ascends, and where the polar cells are subsequently formed. That 

 the protoplasm is accumulated here in greater quantity is in part 

 referable to the fact that it comes to the surface of the egg along 

 with the nucleus, which most certainly furnishes a centre of attrac- 

 tion for the protoplasm. 



The insight into the phenomena of the maturation of the egg, as they have been 

 connectedly presented in the preceding pages, has been acquired only by many 

 roundabout ways and after the removal of many misconceptions. As early as 

 the year 1825 PURKINJE, the discoverer of the germinative vesicle in the Hen's 

 egg, found that in eggs which were taken from the oviduct this vesicle had 

 disappeared, and from this concluded that it was ruptured by the contractions 

 of the oviduct, and that its contents (a lympha generatrix) were mingled with 

 the germ. Whence the name vesicula germinativa. Similar observations were 

 made on this and other objects by C. E. v. BAER, OELLACHER, GOETTE, 

 KLEINENBERG, KOWALEVSKY, REICHERT, and others. But on the other hand 

 the positive statements were made for many eggs (by JOH. MULLER for 

 Entoconcha mirabilis ; by LEYDIG, GEGENBAUR, and VAN BENEDEN for 

 Kotifers, Medusae, etc.) that the germinative vesicle did not disappear, but 

 remained and gave rise by direct division at the time of segmentation to the 

 daughter-nuclei. 



There were therefore in previous decennia two opposing parties : the one 

 asserted the continuance of the germinative vesicle and its division during the 

 process of cleavage ; the other maintained that the egg-cell in its development 

 passed through a condition without nucleus, and again acquired a nucleus in 

 consequence of fertilisation. 



The controversial points were cleared up by investigations which BttTSCHLl 

 and the author had undertaken at the same time. 



I showed in my first " Beitrage zur Kenntniss der Bildung, Befruchtung 

 und Theilung des thierischen Eies," that in all the older writings there 

 had been no distinction made between the nucleus of the immature, the 

 mature, and the fertilised egg, but that these nuclei had been often confounded 

 and held to be identical, and I first established the differences between germi- 

 native vesicle, egg-nucleus, and cleavage-nucleus, the latter being the names 

 which were introduced by me. In addition I showed that the disappearance 



