n8 AGE, GROWTH, AND DEATH 



We have first an embryo of twenty hours of incu- 

 bation, No. i; following it one of one day, No. 2. 

 You can observe just a little line of structure indi- 

 cated and showing where the longitudinal axis is to 

 be situated. By the second day, No. 3, the chick has 

 distinctly a head and a little heart, and those who are 

 expert can differentiate with a microscope the axis of 

 the body, the beginning of the formation of the intes- 

 tine and of the muscles. At the end of the first day 

 there was little more than a mere gathering of cells, 

 but during the twenty-four hours of the second day the 

 gathering has changed from a mere streak upon the 

 surface of the yolk to a well-formed individual, with 



which showed very clearly the progress of development in the chick during the 

 very early stages. The first figure illustrated a chick of eighteen hours' in- 

 cubation. The embryo had been skimmed off from the surface of the egg, 

 hardened, coloured artificially, and mounted in the manner of the ordinary 

 microscopical preparation in Canada balsam. At this age the naked eye can 

 just distinguish a line, which indicates the position of the axis of the embryo. 

 The unaided eye can recognise nothing more. In the second picture the head 

 and neck of the embryo were easily distinguishable, and a few of the earliest 

 primitive segments. The third slide showed a stage of a day and a half. The 

 spinal cord and brain were distinctly differentiated, and numerous so-called 

 " blood islands" scattered about. The final slide of the series showed a chick 

 of three and one half days. It has not seemed necessary to reproduce these 

 figures with the present text, as they merely duplicate, on a larger scale and 

 with more detail, the pictures which have been included. 



EXPLANATION OF FIG. 42. 



TEN STAGES OF THE DEVELOPING CHICK, after Franz Keibel's Normentafeln. 

 All the figures are magnified four diameters. In No. i only the parts indicated 

 in the vertical axis of the figure correspond to embryonic structures proper. 

 No. i. Incubated 20 hrs. No. 6. Incubated 3 days, 16 hrs. 



No. 2. " 24 hrs. No. 7. " 4 days, 8 hrs. 



No. 3. " 2 days. No. 8. " 5 days, I hr. 



No. 4. " 2 days, 19 hrs. No. 9. " 7 days, 4 hrs. 



No. 5. " 2 days, 22 hrs. No. 10. " 8 days, i hr. 



