II. THE DEVELOPMENT AND GROWTH OF THE LAYERS 



OF THE BLASTODERM 1 . 



With Plate I. figs, i 5 and 9 12. 



THE following paper deals with the changes which take place 

 in the cells of the blastoderm of the hen's egg during the first 

 thirty or forty hours of incubation. ,The subject is one which 

 has, as a general rule, not been much followed up by embryo- 

 logists, but is nevertheless of the greatest interest, both in refer- 

 ence to embryology itself, and to the growth and changes of 

 protoplasm exhibited in simple embryonic cells. I am far from 

 having exhausted the subject in this paper, and in some cases I 

 shall be able merely to state facts, without being able to give 

 any explanation of their meaning. 



My method of investigation has been the examination of 

 sections and surface views. For hardening the blastoderm I 

 have employed, as usual, chromic acid, and also gold chloride. 

 It is, however, difficult to make sections of blastoderms hardened 

 by this latter reagent, and the sections when made are not in all 

 cases satisfactory. For surface views I have chiefly used silver 

 nitrate, which brings out the outlines of the cells in a manner 

 which leaves nothing to be desired as to clearness. If the out- 

 lines only of the cells are to be examined, a very short immersion 

 (half a minute) of the blastoderm in a half per cent, solution of 

 silver nitrate is sufficient, but if the immersion lasts for a longer 

 period the nuclei will be brought out also. For studying the 

 latter, however, I have found it better to employ gold chloride 

 or carmine in conjunction with the silver nitrate. 



My observations begin with the blastoderm of a freshly laid 

 egg. The appearances presented by sections of this have been 

 accurately described by Peremeschko, " Ueber die Bildung der 



1 From the Quarterly Journal of Microscopical Science, Vol. xin., 1873. 



