STRUCTURE OF THE OVUM. 181 



contractile vacuoles as existing in the interior of the likewise con- 

 tractile germinal spot. From the germinal spot a hollow tubular 

 process is given off, which is lodged in a similar canal of the germinal 

 vesicle. The vacuoles of the germinal spot communicate also with 

 its tubular process. 



It has hitherto been generally accepted, that the ripe mam- 

 malian ovum is nothing but a much-enlarged primordial ovum ; 

 recent observations and opinions, with which I fully agree, 

 permit us to hold, with great probability, that in the ripe mam- 

 malian ovum, besides the principal yolk, a certain quantity of 

 secondary yolk is also present. Pfliiger (84) has in particular 

 pointed out that there are two different constituents of the yolk 

 in the mammalian ovum, of which one, that in young ova is the 

 clearest, immediately surrounds the germinal vesicle ; whilst the 

 other is darker, and constitutes a superficial investment to the 

 former. We may easily conceive this peripherically situated 

 portion of yolk to be a subsequent or secondary formation, and 

 that it may, perhaps, proceed from the follicular epithelium. 

 This circumstance is at any rate of the greatest importance in 

 the consideration of the comparative anatomy of the ovum. 

 The ovum of Birds and of Reptiles has, up to the present time, 

 occupied to far too great an extent the attention of observers 

 in regard to the significance of its several parts. Very super- 

 ficial observation shows to us that in the ripe ovarian ovum, 

 invested by the vitelline membrane, there is a yellow mass of 

 considerable size, the yellow food yolk, which is invested by a 

 thin coating of white yolk, the white food yolk. 



In the natural condition of equipoise of the egg when at 

 rest, a whitish spot, from three to four millimeters in diameter, 

 is constantly to be found at the uppermost part, immediately 

 beneath the vitelline membrane. (See J. Oellacher, 132.) 

 This tread or cicatricula is the germ or principal yolk with 

 the germinal vesicle. If we examine it in the egg after it 

 has been laid, and has been, as usual, fecundated, it will 

 be found to have already undergone change, and we have 

 before us the furrowed germinal disk. The white yolk every- 

 where surrounds the principal yolk, and penetrates at one 

 point as a small thread into the interior of the yellow yolk, at 



