196 



EMBRYOLOGY 



layer. Upon the top of the funnel-shaped disk of white yolk lies 

 the germinal vesicle, which, like the Avhite yolk, consists of numerous 

 protoplasmatic spherules ; part of the contents of this vesicle 

 shrivels up, and causes the vesicle to assume the shape of a disk, 

 the " germinal disk." The rest, the greater portion of the ovum, so 

 far as it is surrounded by the vitelline membrane, consists of yellow 

 yolk, composed of numerous granular globules of albuminous and 

 fatty matter. 



Thi Ovum in the Oviduct. — The ovum, while still in the u]i})er 

 i:)ortion of the oviduct, is surrounded by the spermatozoa which 

 have worked themselves through the oviduct from the cloaca. 

 They swarm round the surface of the vitelline membrane, and one 

 or more spermatozoa find their way into the germinal vesicle, and 

 fuse with the contents of the latter. Ui)on this impregnation follows, 



S.M. 



DiAGEAMMATIC SECTION OF A FERTILIZED EgG. 



A. Air-chamber at the V>lunt pole ; lil. Blastoderm ; Ch. Chalazte ; S. Shell ; 

 .S.-V. Shell memljrane; Vm. Vitelline membrane. 



while the ovum is still within the' oviduct, the remarkable process 

 of " segmentation." This process consists of the division of the 

 germinal disk by successive cleavages into a number of cells, which 

 step by step build up the complex mechanism of the embryo. This 

 segmentation being restricted to the germinal disk is called " mero- 

 blastic," in opposition to " holoblastic " segmentation, where, as in 

 the ova of the higher Mammals, the whole material of the egg- 

 becomes segmented. 



The egg, having been received into the oviduct, is 2iroi)ellcd in 

 a spiral course by the peristaltic contractions of its walls, and 

 receives from the glands of its lining membrane an accessory mantle 

 of albumen, or the " white " of the egg. The average composition 

 of this albumen is 12 % of proteid matter, 1"5 fat, 0*5 saline 

 matter, and 86 % Avater. The albumen is rapidly added, and 

 o\ving to alternating denser and more watery layers, has a spiral 



