CIRCULATION 663 



vena cava takes the same course as the rest of that which enters 

 the right auricle. 



POSTURE 



During development the posture and proportions of the body 

 undergo changes (Fig. 517). The precocious development of the 

 central nervous system is probably responsible for some of these. 

 It causes the head to be at first disproportionately large and to 

 suffer the cranial flexure (p. 639), which is permanent, and the 

 whole body to assume a temporary ventral curvature which 

 brings head and tail towards one another. As the embryo is 

 raised above the yolk, first the head and then the whole body 

 roll over to lie with the left side downwards. 



HATCHING 



Development lasts for three weeks, and finally, on the twentieth 

 day of incubation, the beak pierces its way into the air chamber 

 which exists at one end of the egg between the two shell- 

 membranes, and the animal begins to breathe by means of its 

 lungs. The shrivelling of the allantois now takes place (the yolk 

 sac has already been absorbed), and the chick breaks its way 

 out of the egg, 



DEVELOPMENT OF MAMMALS 



In all mammals except the Monotremata the egg is minute and 

 undergoes complete and nearly equal cleavage. Its development, 

 however, is very different from that of the similar-looking egg 

 of Branchiostoma. Instead of producing a hollow sphere of cells 

 which invaginates, cleavage nearly always results in a solid, 

 spherical mass or morula ; there is never an invagination, though 

 a stage comparable to the gastrula arises by the establishment 

 of differences between layers of the cells, and possesses, in the 

 primitive streak, the trace of a blastopore. The later course of 

 development resembles in the main that of a bird, a yolk sac 

 (which, however, contains no yolk), an amnion, and an allantois 

 being formed. 



EARLY STAGES OF MAMMALS 



The details of the early stages and of the formation of the embry- 

 onic membranes differ a great deal in different mammals. In 



