436 EMBRYOLOGY OF THE CHICK 



Fig. 250. 



There is a membrane covering the pupil of the eye which, in man, normally dis- 

 appears when the embryo is seven months old. In the case here shown portions of 

 the membrane have persisted as an irregular network over the pupil. Such 

 persistent structures are called rudimentary. (From a drawing lent by Dr. G. N. 

 Brazeau.) 



As we have been using the frog as a norm, or standard type, with 

 which to compare the other forms studied, it would probably seem best 

 to begin embryology with that animal. However, for the same reason 

 that the frog was used as an introductory subject for study (because 

 it can be procured easily and because it is a fairly complex form which 

 possesses structures with which the student is already familiar), so, the 

 hen's egg, which is much larger than that of the frog, can also be pro- 

 cured easily and is already somewhat familiar to the student. And in 

 addition to this, the chick embryo develops upon the surface of the yolk, 

 which makes the various germ layers very distinct, thus serving much 

 better than the frog as a beginning-type. 



The first and foremost point in the study of embryology is accuracy 

 of observation; the second is the obtaining of a clear and understandable 

 concept of what has been observed ; and th'e third point is to show by 

 drawings that the first and second have been fully grasped. 



There is considerable need for legitimate imagination in embryo- 

 logical work, because the entire study of embryology is for the purpose 

 of giving the student a more or less comprehensive idea of the process 

 through which, and by which, all the organ-systems in the body of living 

 things have come to be what they are. The study of embryology is 

 therefore different from our later work in pure anatomy, where each 

 structure is definite, and where such structure is studied only after it is 

 completely formed. In embryology we see the beginnings and develop- 

 ment of these later anatomical structures. 



One should first take the complete embryo, and get a good grasp of 

 the general structure. Then, sections must be cut at various intervals 

 and studied microscopically. It must never be forgotten, however, that 

 our imagination must constantly remind us that there are three dimen- 

 sions to the living animal, and that what we are looking at in our section, 



