658 REPRODUCTIVE ORGANS. 



the chick, while the great body of material nourishes the growing 

 embryo until it leaves the shell and is able to gain its own liveli- 

 hood. As such an embryo is never attached to the parent, it must 

 have within itself, supplemented by what it receives from the air, 

 all the material necessary for its development and maintenance 

 until freed from its enclosing shell, hence the large size of the 

 ovum ; while in the mammal this supply is not necessary, for the 

 attachment to the maternal structures is made at an early period 

 of its history, and from the parent all necessary sustenance is 

 obtained. 



Inasmuch as development has been so much more thoroughly 

 studied in the hen's egg than in any other, and inasmuch as the 

 processes are in many respects probably the same as in the human 

 ovum, the development of the chick will be described, referring 

 to the principal points of difference as they are reached in the 

 description, giving, however, only a general view of the subject, 

 which is much too extensive and complicated to discuss in any 

 other manner in this connection, and referring our readers for 

 fuller details to monographs on embryology. 



Development of Chick. If the shell of a hen's egg is 

 broken during the first day of its incubation and the blastoderm 

 is examined, it will be seen that there is a clear central portion, 

 the area pellucida, and a portion outside of this, the area opaca, 

 which is much less clear. The embryo forms in the area pellu- 

 cida, and the membranes and structures which are to nourish it 

 form in the area opaca. On the second day, the area opaca having 

 meanwhile extended, within it are formed red blood-corpuscles 

 and vessels, and during the same time in the area pellucida the 

 heart is formed. These structures arise, as has been stated, from 

 the cells of the mesoblast. 



At one extremity of the area pellucida a fold forms in the blas- 

 toderm, and, as this is the anterior end, it is called the cephalic 

 fold. A similar fold, the tail fold, forms at the other extremity 

 of the area pellucida. In the same manner lateral folds form on 

 the sides. All these folds, which include the three layers of the 

 blastoderm, approach one another below, and by so doing form a 

 canal, the embryonal sac. This sac is bounded above by the 

 blastoderm, anteriorly by the cephalic fold, posteriorly by the tail 

 fold, and laterally by the lateral folds, while below it is in com- 

 munication with the vitellus. This embryonal sac subsequently 

 becomes divided into two, one division forming the alimentary 

 tract, and the other the body-walls, the umbilicus being the point 

 at which the folds all unite. These folds just described are to be 

 carefully distinguished from the membranes, the amnion, the cho- 

 rion, etc. The folds, as stated, involve the epiblast, the mesoblast, 

 and the hypoblast, while in the formation of the membranes the 

 various layers play different parts. 



