224 



GENEBAL OBNIIUOLQGX. 



the eggs of all birds vary more ia size and shape than some of the devotees of theoreticai oology 

 admit 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 leas 

 variable than the long axis ; for when the quantity of food-yelk and white, upon whiei 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 all the accessory parts of an egg, let us confine attention 

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

 to the female Bynamamaba, consisting of granular protoplasm (vitellus) included in its ceU- 

 wall (vitelline membrane) and including its nucleus and nucleolus (germinal vesicle and germi- 

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

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

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

 result affirmed for mammalian ova, the original germinal vesicle and germinal spot dn,safp;pear, 



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 of 

 evolution, is called 

 a momirvilaa. 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 this duplex or bisexed state, 

 before any further step is taken, the creature is called a cytula, — the parent cell of the entire 

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

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

 but must evolve an entire organism, like both of those two whose vital forces it concentrates, 

 summarizes, and embodies, — or nothing, 



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

 cells, each of which is just like itself. This process is called segmentation of the viteUus; each 

 one of the numerous resulting cells is called a ekavage-cell. The nucleus of the parent-cell 

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



Fig. 111. -Segmentation of the vitellus by discnirial cleavage, diagrammatic, X about 

 10 times, after Haeckel. Only the "tread," cicatvicle, or germ-yelk (figs. 109, b, 110, A) is 

 represented, as no other part of the whole yelk-ball undergoes the process. A^ separation 

 into 2 ; B, into 4; C, into 16, by 8 radial and I concentric furrow ; 73, into many parts, by 

 16 radial and about 4 concentric farrows; E, 64 radial and about 6 concentric furrows ; 

 Fj the whole tread broken up into a mulberry-mass (nuymla) of cells. 



