DIVISION OF THE IMPREGNATED OVUM 



3^7 



The variety of the secretions and movements wholly forbids such an assumption. The secretions and movements 

 are provided by nature for the obvious purpose of completing the egg and transmitting it to its destination out- 

 side the body, where it is to be hatched. The secretions and movements are the result of inherent vital action as 

 apart from every form of irritation and stimulation. As a matter of fact the oviduct takes the lead both in the 

 secretions and the movements. The history of the egg, so far, plainly points to adaptation and design. All the 

 parts involved work spontaneously, consentaneously, and harmoniously to a given end ; that end being the 

 completion, transmission, and discharge of the egg. 



The hatching of the egg is, in a way, as wonderful as its production. 



The impregnated egg can be hatched either by the parent bird, or artificially by means of an incubator. Heat, 

 air, a certain amount of moisture, and turning of the egg at intervals are all that is required. 



extraordinary 

 I 



§ 76. Division of the Impregnated Ovum. 



The changes which occur in the egg during the hatching process are of the most complicated and 

 character. No better example of the stupendous powers and resources of life 

 can be adduced. The germinal vesicle, surrounded by the yolk and vitelline 

 membrane (the original portions of the egg), begins the great process of 

 differentiation by spontaneously dividing into two parts, then into four, then 

 into eight, sixteen, thirty-two, sixty-four, and so on, until it forms a con- 

 tinuous membrane, the so-called blastodermic layer ; the subdivisions 

 assuming a cellular character. The dividing process is still seen in the 

 impregnated human ovum (Fig. 78, D, E, F, G, H). Here again we have 

 an example of spontaneous vital action as apart from irritation and 



stimulation. 



Fig. 77. 



Fig. 77.— Egg of bani-dooi- fowl, consisting of yolk (") ; vitelline membrane (6) ; chalaziferous membrane (c) ; albumen (d) ; 

 internal and middle shell membranes (e, /) ; calcareous shell (g) ; and air chamber (h) (after Dalton). 



Fig. 78 illustrates the history of the impregnated human ovum. 



C BERJt; AU. DEL 



Fii;. 78. 



Fig 78 —A Portion of human ovary with Graafian follicle near the period of rupture. Shows the membrane of the vesicle (0); 

 the membrana granulosa (/>) ; the cavity of the follicle (f) ; the egg (rf) ; the peritoneum (e) ; the tunica albuginea (/) ; and the 



tissue of the ovary (0). ,. , , .,, ,. c , , i, , / n 



B Human ovary with Graafian follicle ruptured. Shows ovum discharged with a portion of the membrana granulosa (a). 



C. Human ovum ruptured by pressure. Shows escape of vitellus and germinal vesicle with germinal spot (rr) 



D. Human ovum, magnified eighty-five diameters. Shows membrana pellucida and vitelline membrane (a) ; vitellus or yolk (b) ; 

 germinal vesicle (c) ; germinal spot (d). 



B, F, G, H. Four different views of the segmentation of the vitellus. 



I. Adult frogs' eggs as they occur in the ovary. 



J. The same when they have passed through the oviduct (after Dalton). 



