DEVELOPMENT OF THE CHICK. 517 



In the region in front of the primitive streak, traces of the embryo 

 begin to appear. The hypoblast becomes in the middle line continuous 

 with the front end of the primitive streak, and defines the future hind 

 end of the embryo. The mesoblast of the embryo originates as two 

 lateral plates split off from the hypoblast, and simultaneously the 

 rudiment of the notochord appears as a median plate. 



Very shortly afterwards, the median line of the epiblast in front of the 

 primitive streak becomes differentiated into a medullary plate, and soon 

 into a medullary groove whose folds will unite to form the medullary 

 canal. 



Before the closure of the medullary canal, the segments or somites of 

 mesoblast begin to be formed. 



As in other cases, the epiblast gives origin to the medullary canal (the 

 brain and spinal cord and their outgrowths), and to the epidermis : the 

 hypoblast lines the alimentary canal, which becomes a closed tube as the 

 embryo is constricted or folded off from the subjacent yolk-sac : the 

 mesoblast is divided into vertebral plates which form the backbone, and 

 lateral plates which form the somatic and splanchnic musculature and 

 other structures. 



It may be interesting, however, to notice some of the steps of progress 

 which characterise the first few days of incubation. 



(1) By the end of the first day of incubation, the changes above 

 described — up to the formation of the first mesoblastic somites — have 

 usually taken place. 



(2) During the second day, the embryo becomes more markedly 

 constricted off from the yolk-sac. "Its long axis is placed at right 

 angles to that of the egg, and the broad end of the egg is on the left side 

 of the embryo." The three vesicles of the brain, the optic outgrowths, 

 the involutions which form the ear, the first hints of cranial flexure, the 

 heart, the growing amnion, are now apparent. 



(3) During the third day, the embryo turns so as to lie partly on its 

 left side, and the internal changes are very important. The cranial flexure 

 is marked, the yolk-sac circulation is completed, four visceral clefts and 

 five visceral arches appear, the optic cup and the lens are formed, the 

 ear-sacs are closed, the nasal pits develop, the parts of the brain are more 

 differentiated, the stomatodseum, mesenteron, and proctodseum form a 

 complete gut, the lungs, liver, and pancreas arise as outgrowths of the 

 mid-gut, and so on. 



(4) Some of the chief events of the fourth day, are the increase of the 

 cranial and body flexures, the formation of the limb-buds, the outgrowth 

 of the allantois from the end of the mesenteron, the appearance of 

 primitive ova in the germinal epithelium. 



(5) During the fifth day, the allantois grows, the chondrocranium is 

 formed, the limbs become more distinct, the auricles are separated, the 

 spinal cord begins to be divided by fissures. 



(6) During the sixth day, the distinctively bird-like characters begin 

 to appear. 



The amnion, which is completely closed on the fourth day, increases 

 in size, and on the seventh day is of very considerable dimensions. The 

 cavity between its folds is filled with fluid, and rhythmical contractions 

 of the amnion rock the embryo to and fro. 



