DESCRIPTION OF THE SEXUAL PRODUCTS. 25 



and the yolk as a mass of enveloping substance. A unanimity of views in this 

 matter was brought about only after the general conception of " cell " had 

 received in Histology a more precise definition. This was due especially to 

 more accurate knowledge of the processes of cell-formation gained through 

 the works of NAGELI, KOLLIKER, REMAK, LEYDIG, and others. 



The interpretation of eggs with separate formative and nutritive yolk, and 

 with partial cleavage, occasioned especial difficulty. Two antagonistic views 

 in this matter have existed for a long time. According to one view, eggs with 

 polar nutritive yolk (the eggs of Reptiles, Birds, etc.) are compound structures, 

 which cannot be designated as simple cells. Only the formative yolk, together 

 with the germinative vesicle, is comparable with the Mammalian egg ; the 

 nutritive yolk, on the contrary, is something new, superposed upon the cell 

 from without, a product of the follicular epithelium. The spherules of the 

 white yolk are explained as uninuclear and multinuclear yolk-cells. The 

 formative and nutritive yolk together are comparable with the entire contents 

 of the GRAAFIAN vesicle of Mammals. H. MECKEL, ALLEN THOMSON, 

 ECKER, STRICKER, His, and others, have expressed themselves in favour of this 

 view with slight modifications in the details. 



According to the opposite view of LEUCKART, KOLLIKER, GEGENBAUR, 

 HAECKEL, VAN BENEDEN, BALFOUB, and others, the Bird's egg is just as truly 

 a simple cell as the egg of a Mammal, and the comparison with a GBAAFIAN 

 follicle is to be rejected. The yolk never contains enclosed cells, but only 

 nutritive components. As KOLLIKER, especially in opposition to His, has 

 shown, the white-yolk spherules contain no structures comparable with genuine 

 cell-nuclei ; and therefore cannot be interpreted as cells. As GEGENBAUE 

 already in 1861 sharply formulated it : " The eggs of Vertebrates with partial 

 cleavage are on that account essentially no more compound structures than 

 those of the remaining Vertebrates; they are nothing else than enormous 

 cells peculiarly modified for special purposes, but which never surrender this 

 their real character." There would be no change in this interpretation, even 

 if it should prove to be that the yolk was formed in part from the follicular 

 epithelium, and was set free from the latter as a sort of secretion. In that 

 event we should have to do with a special method of nutrition of the egg, the 

 cell-nature of which cannot on that account be called in question. 



Various components of the yolk have received special names. REICHERT 

 first distinguished as formative yolk the finely granular mass, which, in the 

 Bird's egg, contains the germinative vesicle, and forms the germ-disc, because 

 it alone undergoes the process of cleavage, and produces the embryo. The 

 other chief mass of the egg he called nutritive yolk, because it does not 

 break up into cells, and because subsequently, enclosed in a yolk-sac, it is 

 consumed as nutritive material. Afterwards His introduced for these the 

 names chief germ and accessory germ (Haupt- und Nebeiikeirn). 



Whereas the nomenclature of REICHERT and His is applicable only to eggs 

 with polar arrangement of nutritive yolk, VAN BENEDEN (1870) has undertaken 

 the division of the substance of the egg from a more general standpoint. He 

 distinguishes between the protoplasmic matrix of the egg, in which, as in 

 every cell in general, the vital processes take place, and the reserve and 

 nutritive materials, which are stored up in the protoplasm in the form of 

 granules, plates, and balls, and which he designates as deutoplasm. Every 

 egg possesses both components, only in different proportions, in varied forms 

 and distribution. BALFOUR has selected this latter condition as a basis f ji 



