DESCRIPTION OF THE SEXUAL PRODUCTS. 15 



The envelopes exhibit, in comparison with the Mammalian egg, an 

 increase in number, for to the zona pellucida (zona radiata), which 

 is produced in the follicle, there is subsequently added still another, 

 a secondary envelope. This is a thick, viscid, gelatinous layer, 

 which is secreted by the wall of the oviduct, and which becomes 

 swollen in water. 



The polar differentiation, taken, as it were, in the very process 

 of developing in the case of the Amphibia, is found sharply expressed 

 in our third example, the Bird 's egg. 



In order to form a correct picture of the condition of the egg-cell 

 in the case of the Hen, or of any k.b k.sch 



other bird, we must seek it while 

 still in the ovary, at the moment 

 when it has finished its growth, 

 and is ready to be set free from the 

 follicle. It is then ascertained that 

 only the spheroidal yolk, the so- 

 called yellow of the egg, which in 

 itself is an enormously large cell 

 (fig. 6a), is developed in the botryoidal ^V^T' 11 <y U ? J ?" ?" 



/' taken from the ovary, k.sch, Gevmma- 



OVary. It is enclosed in a thin but tive disc ; t.&, germinative vesicle ; 



tolerably firm pellicle (d.k), the ^^J^^J^ y * 

 vitelline membrane, the rupture of 



which is followed by an extrusion of the soft pulpy contents. By 

 careful examination one will discover upon the latter a small white 

 spot, the germinative disc (k.sch), or discits proligerus, also called scar 

 or cicatricula. It has a diameter of about 3 or 4 mm., and consists 

 of formative yolk, a finely granular protoplasm with small yolk- 

 spherules, which alone is involved in the process of cleavage. In 

 the flattened germinative disc is also found the germinative vesicle, 

 fig. 6a (k.b) and fig. 6b (a;), which is likewise somewhat flattened and 

 lenticular. 



The remaining chief mass of the egg-cell is nutritive yolk, which 

 is composed of numberless yolk-spherules united by slight traces of 

 egg-plasm, as though by a cement. Information concerning its finer 

 structure is to be gained from thin sections through the hardened 

 egg, which should be cut perpendicularly to the germinative disc. 

 According to differences in staining and in elementary composition, 

 there are now to be distinguished the white and the yellow nutritive 

 yolk (fig. 6a). 



The u-hite yolk (w.d) is present in the egg cell only in a small 



