376 CHORDA TA. 



proximal tarsals to the tibia. The foot moves in the birds 

 upon an inter tar sal joint, the movement being between the 

 two rows of tarsals. 



Development {Gallus). — The true ovum of the fowl is a large yellow 

 sphere enclosed in a delicate vitelline membrane. It is usually termed 

 the "yolk" of an "egg." It is fertilised at the top of the Fallopian 

 tube and passes slowly down the oviduct, developing as it goes, so that 

 a laid ^^ Qgg" has already developed for about eighteen hours. As it 

 passes down the oviduct albumen is added to it from glands of the 

 oviduct, and this is twisted by rotation of the ovum into two cords at 

 the ends of the ovum (chalazae). Further down a double egg-mem- 

 brane and a shell are added and the egg is then laid. 



Segmentation is, as in the skate, meroblastic and produces a small 

 blastoderm resting on the yolk. On laying, the reduction of tem- 

 perature causes development to cease, and in the natural condition 

 it is not resumed till the full complement of eggs has been produced 

 and the. hen commences to "sit." 



Fig. 270. — Three Consecutive Stages of the Blastoderm 

 OF A Chick in early Stages of Incubation. 



(After KoLLER.) 

 Area Opaca. 



Blastopore, 



The blastopore is seen in the first to be crescentic, and is gradually converted 



by differential growth into a longitudinal groove which closes 



to form the primitive groove. 



If sections of the blastoderm be made it will be found, as in the 

 skate, to consist of two layers, epiblast and hypoblast, and a segmenta- 

 tion cavity between them. At the future hind-end, as in the skate, is a 

 thickened rim, immediately behind which a crescentic hole passes into 

 a cavity, the subgerminal (or, possibly, the archenteric cavity). As 

 in the skate, the epiblastic edge of the blastoderm extends gradually 

 round and envelops the yolk by epiboly, but in this case the extension 

 is on all sides, and hence the final closure is effected at the distal 

 pole (opposite to the embryo). In the future posterior region of the 

 embryo the epiblast and hypoblast remain in continuity ; hence the 

 epiblast does not actually extend backwards at this point, but it 

 sweeps round each side, converts the crescentic groove into a longi- 

 tudinal one and completes an even edge beyond it. By the third 



