DEVELOPMENT OF THE CHICK. 



59 



of the egg, while at the umbilicus they are in 

 connection with the young chick. The allantois, 

 with its capillary blood-vessels, thus serves as a 

 temporary lung by which the blood is oxygen- 

 ated from the outer air, the chick not being able 

 to use its true lungs till the very eve of hatching. 

 The allantois is thus a structure of cardinal 

 importance to the life of the growing chick. 



At the tenth or eleventh day (we speak of 

 a hen's egg) signs of the feathers can be dis- 

 tinguished, and motion of the young animal is 

 often perceptible when the egg is opened. 

 Generally about the nineteenth day the beak 

 ruptures the membrane which divides off the air- 

 chamber, and the chick for the first time breathes 

 air through the lungs, after which the chick's 

 blood gradually ceases to flow into the veins of 



replaced by fresh material from the albumen, 

 drawn through the delicate membrane. The 

 albumen comprising much of the bulk of the egg, 

 it is manifest must furnish much of that of the 



Fig. 26. — Seventh Day : allantois, al, more developed. Volk- 

 sac shown in connection with the navel. 



the allantois, which has completed its work and 

 is no more needed. Finally the chick breaks 

 the shell by the aid of a sharp-pointed hard 

 scale, specially provided for the purpose at the 

 tip of the upper beak. It should be noted that 

 the constant tapping sound often heard for the 

 last two days, is not due to this process, but was 

 shown by the late Dr. Horner to be due solely 

 to respiratory action : the breaking of the shell 

 is due to more violent spasmodic movements 

 only made at intervals of five to ten minutes. 

 The arrangement of the chick the day before 

 hatching is shown in Fig. 27. 



During this process of development the 

 embryo has at first been lying as a small object 

 on the upper surface of the yolk ; later on, as 

 it increases in size and definiteness 

 ''"TthT*"' °'' ^°''"^' '^ '^ clearly apparent that 

 Young Chick. '^'''^ neck of the yolk-sac is in con- 

 nection with the umbilicus or navel 

 (Fig. 26). The material needed for growth is 

 therefore derived primaril}' through the yolk; 

 but as the original yolk-matter is absorbed, it is 



Fig. 27. — Nearly ready to hatch. 



chicken ; but it passes through the yolk-sac in 

 this process. Shortly before hatching the entire 

 remaining nutritive material of the egg is 

 gathered within the considerably shrunken 

 yolk-sac, communicating with the umbilicus of 

 the young bird, as in Fig. 28 ; and during the 

 last few hours it is rapidly drawn into the 

 abdomen, where it furnishes food for the newly 

 born chicken during the first day of its indepen- 

 dent e.xistence. In chickens this process is often 

 not complete, a portion of the yolk being still 

 visible outside the abdomen ; and American re- 

 searches have traced this as being generally the 

 result of too great variation in tanperatnre dur- 

 ing incubation, or of too great heat. Such chicks 



Chick two days before exclusion. 



may in spite of this grow up quite strong, the 

 process being completed outside the egg, but 

 as a rule they perish from weakness. The 

 yolk can still be clearly distinguished in most 

 newly hatched chickens, and remains of it can 



