DEVELOPMENT OF GALLUS 199 



pigment which gives the " egg " its characteristic colour and 

 pattern. 



The egg goes down the oviduct with its axis transverse 

 to the long axis of the oviduct, and it is rotated as it descends, 

 with the result that the denser albumen at the two ends, in the 

 axis of rotation, is spirally wound and forms the chalazse. 



■.:.'.'• .: '■':&&•: :~ ■•■:•;'■ •'••>>•• ■ ■ •.'.;'.'■. &<V;. '«■>£»•.*••.• rr V.'.. .-.SS.v. •• . . <-....;• •.'•.'■•V 



: -v-^^^y.;/,^:.:;;;..l ? </ : -. : - ■:.• tew*-- 4 vV/'.' ^l? - :: —*' • ••*••.•••*:•>? 



'»• 



Fig. 87. — Gallus : view of the blastoderm of a hen's egg. (After Jenkinson.) 

 A after 12 hours', B after 18 hours' incubation, as seen by transmitted 

 light, n, notochord ; pa, proamnion ; ps, primitive streak. 



At the blunt end of the egg, the two shell-membranes are 

 separated by a space full of air, the air-chamber. 



Cleavage. — Soon after fertilisation, the process of cleavage 

 begins ; its early stages therefore take place while the egg 

 is descending the oviduct, and it has proceeded some way 

 when the egg is laid. The amount of yolk compared with 

 that of protoplasm is so big that cleavage is incomplete, or 

 meroblastic. While the small quantity of protoplasm at the 

 animal pole divides into a number of cells arranged like a 

 small disc or blastoderm on the top of the yolk, the latter is 



