224 



GENEBAL OBNITUOLOGY. 



the eggs of all birds vary more in size and shape than some of the devotees of theoretical oology 

 iidmit in their practice. The variation so well known in any breed of domestic fowl is scarcely 

 above a normal rate. The short diameter, corresponding to the calibre of the oviduct, is less 

 variable than the long a.xis ; for when the quantity of food-yelk and white, upon which the 

 difference in bulk depends, varies with the vigor of the individual, the scantiness or redundancy 

 is expressed by the shortening or lengthening of the whole mass. The egg traverses the 

 passage small end foremost, like a round wedge, with obvious reference to ease of parturition 

 by more gradual dilatation of the outlet. 



Germination. — Leaving now aU the accessory parts of an egg, let us confine attention 

 to the germ-yelk, or " tread," which is alone concerned in the germinative process. Eecurring 

 to the female Dt/namamceha, consisting of granular protoplasm (vitellus) included in its cell- 

 ■\v'all (vitelUne menrbrane) and including its nucleus and nucleolus (germiual vesicle and germi- 

 nal spot), we wiU trace it up to tlie time it begins to take shape as an embryo chick. At first, 

 as I have observed before, it is like any other amojba ; the first step of development is prob- 

 ably a retrograde one ; for if there ensues, when the spermatozoa melt into tbe ovum, the 

 result alfirmed for mammalian ova, the original germinal vesicle and germinal spot disappear, 



and the whole con- 

 tent of the ovum 

 proper is simply a 

 homogeneous mass 

 of granular proto- 

 plasm. In this ret- 

 rograde step, the or- 

 ganism, at the low- 

 est possible round 

 of the ladder tif 

 evolution, is called 

 a iiionenda. The 

 germinal vesicle 

 and spot, however, 

 are speedily recon- 

 structed, and the 

 ovum looks pre- 

 cisely as it did be- 

 fore. But observe 

 that the actual dif- 

 ference is enormous; 

 for it now consists 

 of the blended sub- 

 stance of the original ovum and of the spermatozoa ; and in tliis duplex or bisexed state, 

 before any further stop is taken, the creature is called a ci/tula, — tlie parent cell of tlie entire 

 future organism. In the former state it could reproduce nothing, not even itself; for it is the 

 strange physiological law of a Bynamanmha that it cannot reproduce like an ordinary cell, 

 liut must evolve an entire organism, like both of those two whose VAal forces it concentrates, 

 summarizes, and embodies,— or nothing. 



Tiie first change in the parent-cell is that by which it becomes broken up into a mass of 

 cells, eacli of which is just like itself This process is called segmentation of the vitellus; each 

 (me of the numerous resulting cells is called a cleavage-cell. The nucleus of the parent-cell 

 divides into two; each attracts its half of the yelk; the halves furrow apart and thcr(^ are now 



rir III s ni tu ti ftl Mt llus 1 ^ lisL'oidal cleavage, itiagraramatic. X about 

 10 tmies alt ill 11 Oi 1\ fl u tr il oiile, or gerni-yelk ifigs. 109, i, 110, ^-1) is 



reprcsentuJ, .IS. II) tlitr p.irt ftlic ub lo jl"^ I'.ill undergoes the process, ^.separation 

 into2; 7i,into4; T, into 16, liy 8 radial and 1 concentric furrow; /J, into many parts, by 

 16 radial and about 4 concentric furrows: E, 04 radial and about C; concentric furrows: 

 F^ the whole tread broken up into a mulberry-mass {morula) of cells. 



