CHAPTER XXIX. 



EMBRYOLOGY OF THE CHICK 



THE DEVELOPMENT OF THE EMBRYO BEFORE THE 



EGG IS LAID 



Before beginning the work proper in Introductory Embryology ii 

 is quite essential that the student turn back to earlier Chapters and re- 

 read what is said there on mitosis, fertilization, and the histology of the 

 frog. Such a review will lay a foundation for the detailed study of the 

 following pages. 



When we come to take up Comparative Anatomy in the next semes- 

 ter's work, it will be found that the Haeckelian law of biogenesis (also* 

 called the "recapitulation theory"), although untrue in its usual applica- 

 tion, is a very convenient supposition in that it makes many points clear 

 if we accept it as a working hypothesis. This so-called law is denned 

 as follows : All animals, during their embryonic period, pass through 

 the same adult-stages that the various members of the race to which 

 they belong have passed. For practical purposes it is necessary to keep 

 this theory in mind in the study of embryology; for, it is the simplest 

 way of bringing home to the student the fact, that in any biological study 

 that is to be scientific, one must first study the more simple organisms 

 and then compare such simple forms with those that are more complex 

 the so-called higher forms. 



All living animals pass through a quite similar stage of development 

 in their embryonic period, so that the next succeeding higher form prac- 

 tically possesses everything that the immediately next succeeding lower 

 form possesses, plus something additional. And it is this "plus some- 

 thing" that we are trying to arrange in proper order when we study 

 embryology. 



The value of this is not always clear to the student, but if he will 

 remember that a human being and a chick pass through quite similar 

 stages during their embryonic periods, the human being, however, devel- 

 oping further, he can understand how an obstruction may prevent any 

 individual part of an organism from receiving the proper nourishment 

 and environment, and thus cause such part to cease developing, thereby 

 producing what is called a rudimentary structure. (Fig. 250.) 



Now, while all animals differ slightly from each other, there are 

 certain type-forms, in which the greatest differences can be clearly ob- 

 served. Such type-forms as commonly used in the laboratory are the 

 dog-fish, as a representative of the cartilaginous fishes; the frog, as an 

 example of amphibia; the chick or pigeon, as an example of birds; the 

 turtle, as an example of reptilia ; and the cat, rabbit, or pig, as an exam- 

 ple of the mammals. 



