270 THE BIOLOGY OF BIRDS 



at the time of hatching, and it does show a progressive 

 atrophy after the chick is hatched, to the eighth day at least. 

 But the state of the muscle is peculiar before the time of 

 hatching. It is infiltrated with lymph and this is associated 

 with the complete incorporation of the yolk-sac and with the 

 discharge of blood from the vessels of the allantois, whose 

 function is over, into the systemic vessels of the chick. The 

 infiltrated musculus complexus is physiologically incapaci- 

 tated from pronounced muscular contraction, so that it 

 cannot be instrumental in breaking open the prison doors. 



According to Pohlman, the active agents in breaking the 

 shell are chiefly the musculus biventer and the musculus 

 spinalis, which move the head and the upper vertebrae of 

 the neck. During the late stages of development the head 

 is to be seen bent to one side ; the muscles in question shift 

 it into one of extension more nearly in line with the vertebral 

 axis. This straightening of the head is responsible in great 

 part for breaking the egg-shell. 



Furthermore, the stimulation of the muscles referred to 

 is probably due indirectly to the distension of the abdomen 

 brought about by the complete incorporation of the yolk-sac. 

 This pulls the trigger of a drinking-choking reflex, which is 

 not dependent on a demand for oxygen. It seems that the 

 whole body of the chick suffers from (cedematous) excess of 

 water. This is due to the changes in the yolk-sac and the 

 allantois, to awakened glandular activity, and to insufficient 

 kidney elimination. When hatching makes it possible for 

 pulmonary respiration to begin, the excess of water is soon 

 got rid of, for that is one of the functions of the lungs in birds. 

 The facts also account for the ability of the newly-hatched 

 chicks to go at least three (perhaps four or five) days without 

 water. 



Re-investigation of the process of hatching should be 

 made in the light of Pohlman 's criticism, but it looks as if 

 the important muscular agency was to be found not in the 

 complexus but elsewhere, and that the breakage of the shell 

 is due to efforts to straighten out the head, which has hitherto 

 been bent to one side. 



