DEVELOPMENT OF THE EMBRYO 437 



is but a series of still pictures, and that there is little value or meaning 

 in such observation unless one can, with imagination and logic, plus 

 preceding biological knowledge, build up a completed structure, so that 

 the mind's eye can see the entire animal as it actually is. 



It must be remembered at this point that events which have already 

 taken place in the past, are the cause or causes of events that are now 

 taking place, and that will take place later. This is as true in embry- 

 ology as it is in such a study as history, for example. This means that 

 the various events of development are caused by preceding develop- 

 mental events and that these cause later steps in development in turn. 



Another important point for the student to remember is that he 

 must not only be able to recognize histologically the type of cells he 

 may find in the section he is studying, but he must know the definite 

 location in the complete embryo from which his section is cut. 



The complete bird-like form of the chick can be clearly seen before 

 the eighth day of incubation, all the principal changes having taken 

 place by that time. It will, therefore, be understood that these changes 

 are rather minute in their origins, for the eight-day embryo is only about 

 seven milHmeters in length. During, and after the eighth day,- the 

 changes which take place are primarily enlargements, or growth of por- 

 tions already present. 



In the study of embryology we are not only interested in the devel- 

 opment of the chick from the egg, but we also wish to know how the 

 egg came, into existence. 



The hen's egg is usually said to be a single cell. This is, however, 

 only true if the egg is unfertilized. 



As birds' eggs are laid with shells upon them, it is necessary that 

 fertilization take place before the shell is formed. Fertilization in these 

 cases is internal. It takes about twenty-two hours for the egg to have 

 the various layers of yolk and white laid down, and for the shell then 

 to surround it. If the egg has been fertilized, the warmth of the mother's 

 body has already caused development throughout these days, so that 

 by the time the egg is laid, the little chick is already approximately two, 

 or two and a half, days of age. There is a variation in the age because 

 if the hen's egg is ready for laying during the main part of the day, it 

 is laid then, but if it is not ready for laying until, let us say, about four 

 or five o'clock in the afternoon, it is retained within the mother's body 

 until the following day, thus causing some embryos to be developed from 

 ten to fifteen hours more than others. 



The so-called spoiling of eggs is usually due to the embryo chick 

 dying and then decaying. 



In birds, where the eggs leave the mother's body, the yolk must 

 be quite large in order to furnish sufficient food for the embryo during 

 the two or three weeks intervening between the time the egg leaves the 

 mother's body and the time of hatching. In the viviparous animals, the 



