DESC niPTl- \ M ! (TS. 



and the yolk as a ma inaniinity ; this 



matter was brought about only att-r tin- -neral concept ion of cell 



.( (1 in Histology a more ! I 



more accurate knovvle d-v of the pi .-'-ll-format ion -.rained tlirough 



irks Of NAGBLI, K"|.I.IKI:I:. KIMAK. !.I:YI>I<;. and others. 



The interpretation :th separate formative and 1111' .. an<l 



with partial oles .(M.iiied especial dillieulty. Two anta^'"' 



in this matter have e:;i-ted for a Ion- time. Aee,,rding to one vic\v, eggs with 

 polar nutritive yolk (the eggs of Reptiles, P.inls, etc.) an- compound structures, 

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

 with the LTenninat ive \esicle, is comparable with the Mammalian egg; the 

 nutritive yolk, on the contrary, is something ne\v, superposed upon the cell 

 from without, a product of the follicular epithelium. The split-rules of tin- 

 white yolk are explained as uninuclear and multinudear yolk-cells, 

 formative and nutritive >olk together are comparable with the entire QO1 

 of the GKAAFIAN vehicle of Mammals. II. MI:<-KI:I., AI.I.KN THOU 

 K< KU;. >TI:ICKI-:R, His, and others, have expressed themselves in favour of th-s 

 \ie\v with slight modifications in the details. 



.According to the opposite view of LEUCKART, KOLLIKER, GE(;.::NBAI i:. 

 HAEC-KKL. VAN |'I:M-:DI-:.V, BALFOUB, and others, the Bird's egz is just as truly 

 a simple cell as the egg of a Mammal, and the comparison with a CIJAAI i \v 

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

 nutritive components. As KOLI.I K 1:1:, especially in opposition to His, has 

 shown, the white-yolk spherules contain no structures comparable withgenuine 

 cell-nuclei: and thnvl'.>re cannot be interpreted as cells. As GEGENBAUR 

 already in 1M>1 sharply formulated it : "The eggs of Vertebrates with partial 

 cleavage are on that account essentially no more compound structures than 

 of the remaining Vertebrates: they are nothing else than enormous 

 cells peculiarly modified for special purposes, but which never sun 

 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 >hould have to do with a special method of nutrition of t! 

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



Various components of the yolk have received special name-. KI:ICII 1:1: T 

 lir.-t distinguished as formative yolk the finely granular mas>, which, : 

 I'.ird's egg, contains the germinative vesicle, and forms the germ-disc, be< 

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

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

 break up into cells, and because subsequently, enclosed in B 

 consumed as nutritive material. Afterwards His introduced for these the 

 names chief germ and accessory germ (Il/mpt- und XebfnL 



Whereas the nomenclature of KEICHKKT and His is applicable only to 

 with polar arrangement of nutritive yolk, VAN I !>7<>)has under 



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

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

 every cell in general, the vital pn>c, s>es take place, ;md the reserve and 

 nutritive material-, which are stored up in the j in the form of 



granules, plates, and balls, and w'.:i -h be designates a- deutOplasiIL i 

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

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



