624 REPRODUCTION. 



its transparency and well defined outline, and marked, in the ovarian 

 egg, by a germinative spot. According to Kolliker, the germinative 

 spot disappears before the mature yolk is discharged from the ovary ; 

 and it is consequently not visible in the egg in the oviduct. 



The cicatricula is the only part of the fowl's yolk which undergoes 

 segmentation, and which is directly concerned in the production of the 

 embryo. It corresponds therefore with the vitellus of the mammalian 

 egg, and has received the name of the " plastic" or formative vitellus; 

 while the remainder, consisting of the white and yellow yolk, is known 

 as the " nutritive " vitellus. The position of the cicatricula is imme- 

 diately above the tubular prolongation of white yolk leading to the 

 central cavity of the egg. 



Segmentation in the FowVs Egg, and Formation of the Blastoderm. 

 The fowl's egg is fecundated soon after leaving the ovary, in the 

 upper portion of the oviduct. Segmentation begins in the lower half 

 of the oviduct and goes on during the production of the shell mem- 

 branes and shell ; and in the new-laid egg the formation of the blasto- 

 derm is usually complete. 



The process of segmentation in the fowl's egg differs from that 

 already described (page 61T) in the following particulars : Instead of 

 a globular vitellus successively bisected into smaller spheres and hemi- 

 spheres, there is a flattened vitelline disk, the cicatricula, which is cut 

 by superficial furrows, running in various directions, and dividing its 

 area into a number of spaces by their intersection. The principal fur- 

 rows radiate from the central part of the cicatricula, and are united at 

 irregular intervals by cross furrows, which mark off isolated portions 

 of its substance. The cicatricula is thus broken up into a large number 

 of segments; but this segmentation takes place by extension over a flat- 

 tened surface, spreading gradually from the centre outward, instead of 

 affecting at once the whole vitellus, as in the mammalian egg. 



The details of segmentation in the fowl's egg have been most fully 

 studied by Coste* and Kolliker.-f It begins by the appearance of 

 a straight or curvilinear furrow, crossing the middle portion of the 

 cicatricula without reaching- to its edges, and dividing it imperfectly 

 into two nearly equal halves (Fig. 182, I.). This furrow is afterward 

 crossed at right angles by a second, dividing the disk into four sec- 

 tors (Fig. 182, II.). The point, however, at which the sectors meet 

 is not usually the exact centre of the cicatricula, but a little on one 

 side ; and the whole process of segmentation, according to Kolliker, 

 goes on in such a way that its point of greatest activity is always 

 somewhat eccentric in position. The primary furrows thus formed are 

 followed by others which radiate toward the edges of the cicatricula, 

 while its central parts are broken up, as above described, into smaller 



* Histoire Gnrale et particulire du Developpement des Corps organises. 

 Paris, 1847-59. Poule, PI. ii., Figs. 7-13. 

 f Embryologie. Paris, 1879, p. 63-85. 



